Objectives: To characterize recent HIV seroconverters in a sexually transmitted disease (STD) clinic population, and examine changing transmission patterns. Methods: We conducted a case-control study nested within a retrospectively defined cohort of individuals attending Baltimore STD clinics between January 1988 and July 1990. Seroconverters, who tested HIV-positive after having a negative test, were compared to both HIV-negative controls, who were also tested twice, and a second, prevalent HIV-positive control group. Controls were matched 2:1 by sex, clinic, and month of HIV test. Results: Forty-nine out of 6175 (0.79%) patients tested at least twice had documented HIV-1 seroconversion. On multivariate analysis, seroconversion was significantly associated with self-reported injecting drug use [odds ratio (OR), 7.3; 95% confidence interval (CI), 2.3-23)], with being a man who has had sex with other men (OR, 3.5; 95% CI, 1.2-10), or with having sex with a known HIV-infected person (OR, 11; 95% CI, 1.3-96). Thirty-five per cent of seroconverters did not report a risk for HIV infection, and a higher proportion of recent seroconverters also reported no risk. Compared to the prevalent positive control group, more seroconverters reported no risk and a lower proportion reported recognized risks. A diagnosis of gonorrhea was also significantly associated with seroconversion (OR, 2.5; 95% CI, 1.1-5.7). Conclusions: These data suggest increasing heterosexual transmission of HIV in this inner-city STD clinic population. Incident STD, in particular gonorrhea, may increase a patient's risk for HIV infection, suggesting that patients with STD should be targeted aggressively for HIV prevention activities.