The study aimed to estimate the prevalence of unprotected sexual relations and associated factors, by sex, among crack users in Pernambuco. A cross-sectional study was developed in a consecutive sample of 1,062 crack users from Pernambuco, Brazil between August/2014 and August/2015. The effect of factors associated with inconsistent condom use was estimated using multiple regression. The prevalence of HIV infection among women (8.1%) was double that among men (4.3%). Inconsistent condom use occurred in 77.8% of women and 74.7% of men. There were differences between the sexes for factors associated with unprotected sexual intercourse. Among men, inconsistent use was related to programmatic vulnerability, such as having access to condoms. Among women, social vulnerability was decisive, including factors related to religion. These differences suggest specific strategies for preventing Sexually Transmitted Infections.