Predictors of serostatus disclosure were identified among youth living with HIV pre- and post-introduction of highly active antiretroviral therapy (HAART). Two cohorts of HIV-positive youth, aged 13-24, in 1994-1996 (n = 351) and 1999-2000 (n = 253) in Los Angeles, New York, San Francisco, and Miami were sampled through medical providers and a variety of social service agencies. Data were collected on demographic, social, medical, and behavioral topics. Men who had sex with men were more likely to disclose serostatus to their partners. Moreover, a positive association with length of time since diagnosis and the likelihood of disclosure exists; across time, youth were less likely to disclose serostatus to casual partners or HIV-negative partners. Post-HAART, number of sex acts with a partner was associated with increased likelihood of disclosure. Interventions for HIV-positive youth must improve disclosure to casual and serodiscordant sexual partners.