Objective: To examine the transmission behavior among youth living with HIV (YLH), pre- and post-HAART. Methods: Two cohorts were recruited: (1) 349 YLH during 1994 to 1996 and (2) 175 YLH during 1999 to 2000, after the wide availability of HAART. Differences in sexual and substance-use risk acts and quality of life were examined. Results: Post-HAART YLH were more likely to engage in unprotected sex and substance use, to be more emotionally distressed, and to have lower quality of life than were pre-HAART YLH. Conclusions: Targeted interventions for YLH that address reductions in transmission acts and aim to improve quality of life are still needed.