Perhaps the largest limitation to this study is that study comparisons of morbidity and mortality rates were constrained by the wide variation and quality of the study designs. This is particularly evident in the wide range of mortality estimates among the different studies. In general, the incidence estimates were higher when the more standardized WHO case definition was used compared with estimates obtained from physician diagnosis; however, there was significant variability even among studies that used similar case definitions. The use of a standard pneumonia case definition that is designed for surveillance and epidemiologic research would improve generalizability and could allow for direct comparisons of incidence and mortality estimates in China and elsewhere [15, 53] . Most studies spanned multiple years, which would account for differences in seasonality of pneumonia, but a few were conducted for one year or less. Although a few of the studies reported large surveillance populations, many calculated incidence based on relatively small populations or did not report the population under surveillance. Most of these studies were conducted in large, urban centers primarily serving residents of densely-populated areas; few included adults or populations from rural western China. Although many of the studies were conducted prospectively, none calculated incidence using active, population-based surveillance and population denominators were not reliably measured in each study. Until standardized case definitions and appropriate surveillance methodology are applied, pneumo-nia incidence and mortality estimates should be interpreted cautiously.