Since crack cocaine appeared in urban areas in the United States in the mid-1980s, reports have suggested that crack smokers may be at increased risk of sexually transmitted diseases (STDs), including infection with HIV, because they have multiple sex partners, trade sex for money or drugs, and rarely use condoms. A cross-sectional survey is being conducted in urban neighborhoods in Miami, New York and San Francisco--where crack use is common--to explore these issues. Indigenous street outreach workers are recruiting men and women who are either current regular crack smokers or who have never smoked crack; each group is further stratified according to whether participants had ever injected drugs. Participants were interviewed about their sexual and drug-use practices. Overall, crack smokers, whether injectors or not, engaged in higher-risk sexual behaviors than nonsmokers, reported greater numbers of sex partners than nonsmokers, and were more likely than nonsmokers to have exchanged sex for money or drugs or to have had an STD. Differences between crack smokers and nonsmokers were generally greater among non-injectors than among injectors, and generally greater among women than among men. Condom use, although somewhat more common with paying than nonpaying partners, was infrequent overall. Most of the subjects had not been in substance abuse treatment in the preceding 12 months, and a majority had never been in substance abuse treatment. Education and prevention programs specifically targeted at crack smokers not currently in substance abuse treatment are needed to reach these high-risk persons.