Six bundred homeless men and 300 homeless women in St. Louis were systematically interviewed using the revised Diagnostic Interview Schedule that includes a module for assessment of posttraumatic stress disorder (PTSD). Most subjects with PTSD had an additional life time psychiatric diagnosis. No consistent pattern of association was apparent, however, between individual diagnoses and either traumatic events or PTSD. In almost three-fourths of both men and women, the onset of PTSD had peeceded the onset of homelessness. Childhood histories of abuse and family fighting were predictive of both traumatic events and PTSD. The results suggest that factors leading to PTSD in the study sample began long before the onset of homelessness and may overlap with factors operative in the genesis of homelessness.