1. Background  Substance abusers are at risk for HIV and other STIs.  Anal intercourse (AI) is riskier than vaginal intercourse.  Studies of AI have focused primarily on men who have sex with men (MSM). However, recent national surveys indicate that the total acts of AI occur more frequently among heterosexuals than among MSM.  Little has been reported about heterosexual anal intercourse (HAI) among men and women substance abusers.  HAI is riskier for women than men because they are always the receptive partner. Conclusions  More men and women in substance abuse treatment practice HAI than in the general population.  Men engage in HAI more with casual partners than do women.  Condom use for HAI is infrequent.  Younger age, bisexual behavior, being White and having more sex partners (men) were associated with engaging in HAI.  An increased focus on HAI in HIV prevention efforts with men and women in substance abuse treatment is warranted. Acknowledgements This study was supported by National Institute on Drug Abuse (NIDA) grants U10 DA13714 (Dennis Donovan, PI), U10 DA 13035 (Edward Nunes, PI), U10 DA 013727 (Kathleen Brady, PI). The authors wish to thank Paul Crits-Christoph, Ph.D., University of Pennsylvania and Robert Gallop, Ph.D., West Chester University, for their assistance in combining the datasets from CTN protocols 0018/0019. No conflict of interests to report. Gender differences of heterosexual anal sex among men and women in substance abuse treatment Donald A. Calsyn, Ph.D. 1,2, Mary A. Hatch-Maillette, Ph.D. 1,2, Christina S. Meade, Ph.D. 3, Susan Tross, Ph.D. 4, Aimee N. C. Campbell, Ph.D. 4 1 Alcohol & Drug Abuse Institute, 2 Department of Psychiatry & Behavioral Sciences, University of Washington, Seattle, WA, 3 Duke University, Durham, NC, 4 Columbia University College of Physicians & Surgeons, New York, NY Methods  Participants were men (n=539) and women (n=421) enrolled in the NIDA Clinical Trials Network (CTN) “Real Men Are Safe” and “Safe Sex Skill Building for Women” trials who reported heterosexual activity at baseline.  A logistic regression model with HAI as the DV included variables related to high risk sex, and participant sex as an interaction term. Results – Condom Use  Men more likely than women to use condoms for HAI with main partners (21.9% vs. 9%, Χ 2 =5.60, p =.02, Figure 3).  No gender difference in condom use for HAI with casual partners (Figure 3).  No differences in  Percent of total sexual intercourse events that were HAI (Median=16.67 men vs. 13.04 women).  Percent who engaged in HAI with their main sexual partner. (Figure 2).  Percent of men (4.3%) and women (6.8%) reporting bisexual activity in the prior 90 days.  For men and women, recent bisexual behavior, and being younger and white, were significantly associated with HAI.  For men, number of sex partners was also associated with HAI. Figure 1 Figure 2 Figure 3 Table 1 Specific Aims  Compare the prevalence of HAI among men and women in substance abuse treatment to data from nationally representative sample of U.S. adults.  Compare rates of HAI with main and casual partners among men and women substance abusers in treatment  Identify variables associated with HAI among substance abusers. Results – HAI Frequency  Compared to the general population (National Survey of Sexual Health and Behavior ), more male and female substance abusers engaged in HAI (see Figure 1). Among male and female substance users:  More men (32.8%) than women (27.1%) engage in HAI (Χ 2 =3.7, p =.47, Figure 1).  HAI with casual partners more likely in men (Figure 2).  Of men (n=65) and women (n=37) with both main and casual partners and who engaged in HAI, men (56.9%) were more likely than women to engage in HAI with both main and casual partners (17.6%, Χ 2 =14.0, p <.001).  More bisexual men (52.2%) and women (46.7%) reported HAI in the prior 90 days than heterosexual- only men (32.0%, Χ 2 =4.07, p=.04) and women (25.5%, Χ 2 =6.33, p =.01). Results – Logistic Regression Logistic regression summary identifying variables associated with heterosexual anal intercourse (n=945).