Of 970 sexually transmitted disease (STD) patients enrolled at the Comprehensive Health Centre, Kingston, Jamaica, between November 1990 and January 1991, 710 (73%, 333 men and 377 women) were reexamined between January 1992 and July 1993 to estimate the incidence of HIV and HTLV-I infection and to identify risk factors for infection. Of those reexamined, 20% were recruited passively when they returned to the clinic of their own accord, and 80% were recruited actively through field visits to their homes. Passively recruited persons were significantly more likely than active recruits to have had a sexually transmitted disease since enrollment or at their follow-up visit. Seven men and one woman became HIV positive during the period of follow-up. The overall HIV incidence rate was 0.7 per 100 person years (95% confidence interval [CI] = 0.3 to 1.4), 1.4 (CI = 0.6 to 2.8) for men and 0.2 (CI = 0.004 to 0.9) for women. Four of 270 men and 4 of 318 women were HTLV-I positive, an overall incidence of 0.9 per 100 person years (CI = 0.4 to 1.7), 1.0 for men and 0.8 for women. HTLV-I infection was associated with an age of 30 years or older (p < 0.01). The presumed lower transmission probability for HTLV-I may combine with a higher prevalence of HTLV-I in sexual partners to produce similar overall incidence rates for the two infections. The HIV and HTLV-I incidence rates may have been underestimated, because the study subjects who did not return to the clinic may have had a somewhat higher risk. On univariate analysis, there were significant associations between HIV infection in men and drinking alcohol before sex, cocaine use, total number of sex partners, sex with a prostitute since enrollment, ever accepting money for sex, the average number of sex partners per month, bruising during sex, and genital ulcers found on follow-up examination. This analysis needs to be interpreted with caution in view of the small number of seroconverters, which did not allow testing for independent effects in a logistic regression model.