Objective: To carry out a preliminary assessment of the use of a psychiatric screening tool in an urban homeless population, and to estimate the potential prevalence of undiagnosed and (or) unmanaged mental illness in this population. Methods: Participants ( n = 166) were recruited from the Calgary Drop-in and Rehab Centre to complete a questionnaire containing 6 modules screening for common psychiatric disorders. Summary statistics were used in the analysis. Results: Only 12 respondents (7%) screened negative on each of the 6 modules. The screening process determined that 60.2% of the sample ( n = 100) had probable mental illness but reported no history of psychiatric diagnosis or treatment. Conclusions: A straightforward application of screening (in which screen-positive subjects are referred for assessment) would be difficult in this population as most will screen positive. The results highlight the tremendous burden of psychiatric symptoms in this population.