Fitzpatrick-Lewis et al. BMC Public Health 2011, 11:638 http://www.biomedcentral.com/1471-2458/11/638 housing (NH; n = 66), abstinence-contingent housing (ACH; n = 63), or non-abstinence-contingent housing (NACH; n = 67) [87,88]. Urine was tested to determine any use of cocaine, marijuana, or alcohol during the study. NH participants received no program-provided housing regardless of urine test results. ACH partici- pants received rent-free housing after two consecutive drug-negative urine tests. Participants in the NACH group received rent-free housing in similar buildings after two consecutive urine tests regardless of results. The mean adjusted consecutive weeks of abstinence for the NH, NACH, and ACH groups were 5.28, 4.68, and 7.32, respectively, with statistically significant differences between the NH and ACH groups (p = .024) and the ACH and NACH groups (p = .0031). There were no dif- ferences in mean adjusted consecutive weeks of absti- nence between the NH and NACH groups (p = .51). The number of days housed in the past 60 days from baseline to 12 months increased for all groups (p < .0001). Of note, only 34.1% of participants were stably housed at 12 months since limited housing spaces were available for participants with imperfect abstinence his- tories during the study period [88]. Taken together these data suggest that the provision of housing is an effective intervention for homeless indi- viduals with substance abuse issues, reducing substance use, increasing abstinence, and reducing medical services utilization [83-88]. In addition, abstinence-contingent housing appears to provide greater impact on sustained abstinence than non-abstinence-contingent housing [87,88]. Interventions for Homeless People with Concurrent Mental Illness and Substance Abuse One study examined interventions for homeless people with concurrent mental illness and substance abuse [89-92] (see Table 2). A 48-month longitudinal study examined the effectiveness of Pathways Housing First, a program that offers housing and services to people who are homeless and mentally ill in New York City. Three articles reported on a variety of outcome measures from baseline to 24 months [89,91,92] and at 36 and 48 month follow-ups [90]. A total of 225 participants were randomized into two groups with the intervention group (n = 99) assigned to the Pathways Housing First model that was designed to remove barriers to housing for vulnerable people. This program provided clients with immediate access to independent apartments and support services without the concurrent requirement of sobriety or psychiatric treatment. The control group (n = 126) was assigned to a program called Continuum of Care that provides outreach services and drop-in centres plus congregate living arrangements with support and subsequent placement in independent apartments. Over Page 7 of 14 a 24 month period, the Pathways Housing First group spent 66% less days homeless compared to baseline (p < .001) [89] and demonstrated less need for substance abuse treatment at 36 months (p = .05) [90]. There was no difference in psychiatric outcomes between groups (p = .85) [91], however, the Pathways Housing First group utilized mental health services slightly more than the control group, although differences were only statisti- cally significant at 48 months (specific values not reported) (p = .025) [90]. The methodologically weak studies that examined housing interventions for people with substance misuse issues and mental illness reported similar findings [12,93-96], concluding that the provision of housing and other supportive services may be beneficial interventions. Interventions for Homeless People with HIV Four studies examined interventions for homeless peo- ple with HIV (see Table 2). The first study examined the efficacy of the Healthy Living Program in reducing sexual risk behaviours and substance use among adults with HIV who were marginally housed as a subgroup analysis of a larger (n = 767) sample of adults with HIV [97]. The Healthy Living Program is comprised of three modules, each consisting of five 90-minute individual counselling sessions and addressing the themes of “Cop- ing”, “Act Safe”, and “Stay Healthy”. Individuals were considered marginally housed if they reported being homeless, living in shelters, or living in a welfare hotel during the past 12 months before any assessment inter- view or at the baseline, 15-, 20-, or 25-month assess- ment. These marginally housed participants (n = 270) were randomly assigned to either an intervention group (n = 137) that received the Health Living Program mod- ules or a control group (n = 133) that did not receive the modules but were assessed along the same time lines as the intervention group. The researchers used a Zero Inflated Poisson (ZIP) model that allows for both potential mechanisms of the intervention to be estimated (i.e., whether the interven- tion worked because there was an abstinence from the targeted behaviours or because there was a reduction in the frequency of the acts). Significant effects were found for the average numbers of partners who were HIV- positive or of unknown serostatus (1.8 to 0.56) (p < .001). There was also a significant reduction in the num- ber of days of alcohol or marijuana use (35.77 to 27.54) (p = .002). The intervention also reduced the number of risky sexual acts from 5.03 to 1.75 (p = .037) and the number of days of hard drugs were used during the pre- vious 3 months from 27.76 to 24.00 (p = .042) com- pared to the control group (3.77 to 2.67 and 32.37 to 32.23 respectively). Hard drugs were defined as all