BACKGROUND: Factors influencing the outcome of structured treatment interruptions (STIs) in HIV chronic infection are not fully elucidated. METHODS: In ISS-PART, 273 subjects were randomly assigned to arm A (137 assigned to continuous highly active antiretroviral therapy [HAART]) and arm B (136 assigned to 5 STIs of 1, 1, 2, 2, and 3 months' duration, each followed by 3 months of therapy). Main outcome measures were the proportion of subjects with a CD4 count >500 cells/mm, the rate of virologic failure, and the emergence of resistance at 24 months. RESULTS: The proportion of subjects with a CD4 count >500 cells/mm was higher in arm A than in arm B (86.5% vs. 69.1%; P = 0.0075). Pre-HAART CD4 cell count and male gender were independen...