Antai and Moradi context in which a large number of residents of densely populated areas live in Nigeria. Furthermore, although the spatial concentra- tion of poverty is essential to the definition of disadvantaged neighborhoods, current efforts at systematizing this definition use indicators such as access to safe drinking water, ade- quate sanitation, electricity, overcrowding, and security of housing tenure. The focus is often on households rather than directly tak- ing into account the concentrations of pov- erty or affluence in the neighborhoods that surround these households. Neighborhood effects are a leading example of the forces operating outside households that can exert influence on household-level behavior and health outcomes (Montgomery and Hewett 2004). Thus, there is ample reason, on both substantive and methodological grounds, to explore neighborhood effects of the urban areas of low- and middle-income countries. Nigeria had possibly the fastest urbaniza- tion growth rate in the world in the 1970s (Akinbame and Fadare 1997). Between 1970 and 1980, the proportion of Nigerians living in urban areas was estimated to have grown from 16% to > 20%, and by 2010, urban population is expected to be > 40% of the nation’s total (Akinbame and Fadare 1997). In 1995 Lagos (the former administrative capital of Nigeria) was the world’s 29th larg- est urban agglomeration, with 6.5 million inhabitants, and in 2000 it became the 23rd largest, with 8.8 million people. In 2002, Lagos became one of sub-Saharan Africa’s first mega-urban regions with its metropolitan population reaching 10 million inhabitants. The city continues to grow, and by 2015 it is expected to become the world’s 11th larg- est urban system, with 16 million inhabit- ants (United Nations Human Settlements Programme 2003). As city living becomes the predomi- nant social context for most of the world’s population, the urban environment is bound to shape population health in cities (Galea and Vlahov 2005). Thus, explain- ing the association between urban area dis- advantage and mortality for children < 5 years of age (under-5 mortality) in low- and middle-income countries undergoing rapid urbanization is of importance in developing appropriate health interventions and preven- tive measures for the rising number of urban inhabitants. Under-5 mortality. The under-5 mortality rate is a leading indicator of the level of child health and overall development in countries (McGuire 2006). As such, it is an indicator of the Millennium Development Goals (United Nations 2008), which seeks to reduce the under-5 mortality rate by two-thirds between 1990 and 2015. Under-5 mortality measures child survival and reflects the impact of social, 878 economic, and environmental circumstances as well as other causes of death on infants, toddlers, and young children, including their health care (United Nations Children’s Fund 2007; United Nations Population Fund 2003). Thus, the under-5 mortality rate cap- tures > 90% of the global mortality among children < 18 years of age (United Nations Children’s Fund 2008) and shows large varia- tion across socioeconomic groups and geo- graphic areas and between rural and urban areas. Moreover, data on under-5 mortality are relatively reliable compared with other meas- ures of population health (United Nations Population Fund 2003). We used a multilevel approach to account for the hierarchical structure of the Demographic and Health Survey (DHS) (National Population Commission 2004) data—data for children (level 1) who were clustered within data for mothers (level 2), who were in turn clustered within data for commu- nities (level 3)—because of its suitability for investigating the relationship between area- level socioeconomic disadvantage and mortality using census data or survey data (Bosma et al. 2001; Diez-Roux 2001, 2004; Subramanian 2004). This is based on the notion that area- level characteristics are potential determi- nants of health outcomes and that area-level inequalities may be relevant in the context of increasing geographic clustering of poverty with other forms of disadvantage (Gephart 1997). Although several studies have assessed child survival in urban areas of sub-Saharan Africa, this study is unique in its assessment of the effect of urban area/neighborhood socio- economic disadvantage on under-5 mortality. The aims of this study were a) to assess the trend of urban under-5 mortality in relation to urban population growth in Nigeria and b) to assess whether area-level socioeconomic dis- advantage has an impact on under-5 mortality risks after individual demographic and socio- economic characteristics are taken into account. Methods Cross-sectional data from the 2003 Nigeria DHS were used in this study. This sample was collected using a stratified two-stage cluster sampling procedure. A full report and detailed description of the data collection procedures are presented elsewhere (National Population Commission 2004). Birth history data, such as sex, month and year of birth, survivorship status, and current age or age at death if the child had died were all collected for each of these births. This study was restricted to chil- dren born to the subsample of 2,118 mothers living in urban areas at the time of the survey and to births in the last 5 years before the survey to ensure that the household variables investigated provided a close enough or accu- rate picture of the current living conditions of the children within the period they were exposed to increased risks of mortality. Ethical considerations. This study is based on analysis of secondary data with all par- ticipant identifiers removed. The survey was approved by the National Ethics Committee in the Federal Ministry of Health of Nigeria and the Ethics Committee of the Opinion Research Corporation Macro International, Inc. (ORC Macro Inc., Calverton, MD, USA). Permission to use the DHS data in this study was obtained from ORC Macro Inc. Measures. Outcome variable. The out- come variable was the risk of under-5 mor- tality, defined as a child dying between birth and the fifth birthday. Under-5 mortality was estimated for the 5 years preceding the survey. All children born within the 5 years before the survey date were included in the analysis. Children contributed person-time until they reached 60 months of age or until death or the date of the survey. All deaths among children ≤ 59 months were regarded as cases. Exposure variables. Urban area disadvan- tage was measured using the urban area dis- advantage index (UADI) score. The UADI scores reflect the overall level of urban area disadvantage based on eight indicators of socioeconomic disadvantage at the neighbor- hood level, including the percentage of chil- dren a) living in a household without piped water, b) living in a household without flush toilet, c) living in a household without elec- tricity, d) living in a household without non- polluting cooking fuel, e) whose mothers were unemployed, f) whose mothers were unedu- cated, g) living in crowded households, and h) living in households within the lowest two wealth quintiles (poorest 40%). The UADI scores were generated through principal component analysis at the level of primary sampling units (PSUs). We included 165 urban PSUs in this study out of a total of 365. PSUs or clusters are administratively defined areas used as proxies for “neighbor- hoods” or “communities” (Diez-Roux 2001). These are small, fairly homogeneous units made up of one or more enumeration areas, which are the smallest geographic units for which census data are available in Nigeria. Each clus- ter consisted of a minimum of 50 households, with contiguous enumeration areas added when a cluster had < 50 households (National Population Commission 2004). A similar index has been used in other studies (Barnes et al. 2007; Noble et al. 2006) in the following situations: a) when the main focus of analysis lies in the effects of characteristics of place of residence on health (Macintyre et al. 2002; Whitley et al. 1999), b) to allow for the control of possible socioeconomic confounding when examin- ing the effects of the local environment on health (Macintyre et al. 2002), and c) when ovlume 118 | number 6 | June 2010 • Environmental Health Perspectives