Abstract
Objective
No previous experimental trials have investigated Housing First (HF) in both scattered site (SHF) and congregate (CHF) formats. We hypothesized that CHF and SHF would be associated with a greater percentage of time stably housed as well as superior health and psy-chosocial outcomes over 24 months compared to treatment as usual (TAU).

Methods
Inclusion criteria were homelessness, mental illness, and high need for support. Participants were randomised to SHF, CHF, or TAU. SHF consisted of market rental apartments with support provided by Assertive Community Treatment (ACT). CHF consisted of a single building with supports equivalent to ACT. TAU included existing services and supports.

Results
Of 800 people screened, 297 were randomly assigned to CHF (107), SHF (90), or TAU
(100). The percentage of time in stable housing over 24 months was 26.3% in TAU (refer-ence; 95% confidence interval (CI) = 20.5, 32.0), compared to 74.3% in CHF (95% CI =69.3, 79.3, p<0.001) and 74.5% in SHF (95% CI = 69.2, 79.7, p<0.001). Secondary out-comes favoured CHF but not SHF compared to TAU.

Conclusion
HF in scattered and congregate formats is capable of achieving housing stability among people experiencing major mental illness and chronic homelessness. Only CHF was associated with improvement on select secondary outcomes.