As an ever-increasing number of people infected with HIV are living longer, healthier lives, concerns about continued transmission are growing along with an awareness of the need to develop "prevention for positives." This study of HIV-positive adults in New York City is the first examination of patterns of sexual behavior in a large, representative cohort of HIV-infected individuals followed over an extended time period. A total of 968 HIV-positive adults were interviewed every 6-12 months between 1994 and 2002 and reported considerable variability in sexual behaviors over time. Many persons were not sexually active at all for months at a time; some continued to have multiple partners. Over one third of the cohort had one or more periods when they had engaged in unprotected sex with a partner who was HIV-negative or status unknown (unsafe sex) and one in five reported exchanging sex. Periods of unsafe sex alternated with periods of safer sex. Predictors of sexual risk varied by gender, and among men who had sex withmen, and men sexually active with women only. Contextual factors such as partner relations, housing status, and receipt of HIV services were as important as individual attributes as predictors of unsafe sex and sex exchange. The variability observed in sexual risk behavior reported over time provides new insight into the importance of engaging persons living with HIV in ongoing prevention programs.