Trends in HIV prevalence among young populations (15-24 years) are held to approximate trends in HIV incidence. Using the Serological Testing Algorithm for Recent HIV Seroconversion (STARHS) to estimate HIV incidence, we were able to demonstrate that this assumption was true for young patients at San Francisco's public sexually transmitted infection clinic from 1989 to 1998. However, the trend in prevalence among young people did not mirror trends in incidence in the overall population.