Crack cocaine causes blisters, sores, and cuts on the lips and in the mouths of persons who smoke it, and such sores may facilitate the oral transmission of HIV. We recruited young adults aged 18-29 years, who either were current regular crack smokers, or who had never smoked crack, from inner city neighborhoods in New York, Miami, and San Francisco. Participants were interviewed for HIV risk behaviors and history of recent oral sores and were tested for HIV, syphilis, and herpes simplex virus (HSV) antibodies. Among the 2,323 participants recruited, 1,404 (60%) were crack smokers. Crack smokers (10.0%) were more likely than nonsmokers (4.5%) to report having had oral sores in the past 30 days [prevalence odds ratio (POR) 2.4, 95% confidence interval (CI) 1.7-3.4]. Sores were also more prevalent among those who had ever injected drugs (14.3%) than among those who had not (6.7%; POR 2.3, 95% CI 1.7-3.4), and among those with HIV infection (14.3%) than among those without it (8.0%; POR 1.9, 95% CI 1.3-2.8). Among the 429 participants who reported receptive oral sex, those who reported oral sores were more likely than those who did not to have HIV infection, after other HIV risk factors were controlled for (adjusted POR 1.9, 95% CI 1.0-3.6). Our results confirm that crack smokers have a high prevalence of oral sores and provides evidence that these sores, although infrequently, may facilitate oral transmission of HIV.