Objective: This study compared the prevalence of DSM-III-R disorders among homeless and low-income housed mothers with the prevalence of these disorders among all women in the National Comorbidity Survey. Method: The authors used an unmatched case-control design for assessing 220 homeless and 216 housed mothers receiving public assistance. Results: Homeless and housed mothers had similar rates of psychiatric and substance use disorders. Both groups had higher lifetime and current rates of major depression and substance abuse than did all women in the National Comorbidity Survey. Both groups also had high rates of posttraumatic stress disorder and two or more lifetime conditions. Conclusions: The prevalence of trauma-related disorders among poor women was higher than that among women in the general population. Programs and policies designed for low-income mothers must respond to the high prevalence of DSM-III-R disorders.