One of the goals of this study was to determine how the comorbidity of major depression with substance use disorders impacts the prevalence of suicidal thoughts. Data from the Canadian Community Health Survey: Mental Health & Well-Being were used. Substance dependence & major depressive disorder independently predicted higher prevalence of suicidal thoughts & mental health treatment use. The presence of both major depressive & substance use disorders during the same 12-month period increased the prevalence of suicidal thoughts in a multiplicative way. (24 refs.)