Heterosexual anal intercourse (HAI) is an understudied risk behavior among women and men in substance abuse treatment. Rates of HAI for women (n = 441) and men (n = 539) were identified for any, main and casual partners. More men (32.8 %) than women (27.1 %) reported engaging in HAI in the previous 90 days. These rates are higher than those reported for both men (6.0-15.9 %) and women (3.5-13.0 %) ages 25-59 in the National Survey of Sexual Health and Behavior. Men were significantly more likely to report HAI with their casual partners (34.1 %) than women (16.7 %). In a logistic regression model generated to identify associations between HAI and variables previously shown to be related to high risk sexual behavior, being younger, bisexual, and White were significantly associated with HAI. For men, having more sex partners was also a significant correlate. HAI is a logical target for increased focus in HIV prevention interventions.