Background: Research suggests sexual minority women have higher rates of substance use and mental health problems than straight women. Specifically, past studies have shown alcohol consumption and dependence rates are higher among sexual minority women, in addition to use of some drugs. Similarly, research shows mental health problems such as anxiety, depression and suicide rates are elevated among sexual minority women. These differences in mental health and substance use characteristics by sexual orientation may be explained by the negative health effects of social marginalization and the common use of drinking establishments for sexual minorities. Objective: The objective of this thesis is to compare substance use and mental health characteristics between lesbian/bisexual women and straight women, including: a) demographic variables; b) alcohol and drug consumption and dependence; c) the social context of substance use (i.e., use with others, motivations to use and locations of use); and, d) mental health characteristics. Methods: Data were obtained from a sample of residential treatment clients in treatment for primarily alcohol and/or cocaine problems. Respondents were asked to fill out self-administered questionnaires, which included details on demographics, substance use, mental health and the social context of use, as well as information on sexual orientation and gender identity. Bivariate and logistic regression analyses were performed to examine differences by sexual orientation. Results: Some sexual orientation differences were found regarding alcohol consumption and dependence during bivariate analysis. In logistic regression results, methamphetamine use was significantly (p<.01) elevated among bisexual women and tranquilizers use was elevated among lesbian and bisexual women when compared to straight women. Bivariate analysis revealed lesbian and bisexual women reported higher levels on motivations to use, but this difference was not significant in multivariate regression results. After regression adjustments, lesbian and bisexual women had higher levels of anxiety and higher rates of suicide attempts. Lastly, lesbian and bisexual women reported substance use with sex workers and sex clients more often than straight women, but no other differences in location and motivations to use were seen in the regression results. Conclusion: Among this sample of residential treatment clients, some mental health and substance use characteristic differences were found. These finding can assist in determining the best treatment practices for sexual minority women. / Graduate / 0347