BACKGROUND:Health utility assessments are important for economic evaluations but few instruments have been validated in homeless people with mental illness. We examined the convergent validity of the EuroQol-5 Dimension 3-level questionnaire (EQ-5D-3L) as a measure of quality of life in homeless adults with mental illness. METHODS:Data were from Toronto participants in At Home/Chez Soi, a 24-month randomized controlled trial of Housing First (immediate access to scattered site housing and mental health support services) compared to treatment as usual for homeless adults with a mental disorder (n = 575). Participants completed the EQ-5D-3L at 6 month intervals. We tested convergent validity, hypothesizing strong correlation (r > 0.6) with the Lehman Quality of Life Interview 20 (QOLI-20) index and moderate correlations (r > 0.3) with the Colorado Symptom Index (CSI), Recovery Assessment Scale (RAS), and number of comorbidities. We also examined correlations between EQ-5D-3L scores and the QOLI-20 over time using a linear mixed-effects model. RESULTS:The EQ-5D-3L was not strongly correlated with the QOLI-20 (r ranged from 0.31-0.52 at various time points). The EQ-5D-3L was moderately correlated with the CSI, RAS, and number of comorbidities. The Snijders/Bosker r2 for longitudinal validity between the EQ-5D-3L and QOLI-20 within subjects over time was 0.2094 (square-root r = 0.4576). CONCLUSIONS:The EQ-5D-3L did not demonstrate strong convergent validity in homeless people with mental illness but was moderately correlated with several instruments. Further research is warranted to determine the optimal method for measuring health utilities in this population. TRIAL REGISTRATION:International Standard Randomised Control Trial Registry ISRCTN42520374 assigned on August 18, 2009.