The effect of length of therapy on the safety and efficacy profile of continuous intravenous (CIV) interleukin-2 (IL-2) in combination with antiretroviral therapy (ART) was evaluated in 81 HIV-seropositive patients with CD4(+) T-cell counts of 100-300/mm(3). Patients were randomized to CIV IL-2 (12 mIU/day) for 3, 4, or 5 days plus ART every 8 weeks for six cycles, or to ART alone. The mean percent increase in CD4(+) T-cell counts was 24.5% for IL-2 recipients compared with a mean percent decrease of 30.5% for control patients (P = 0.005). Increasing duration of CIV IL-2 therapy resulted in improved CD4(+) T-cell response. The most frequent clinical adverse events and laboratory abnormalities were predominantly of grade 1 or 2 severity. However, grade 3 or 4 events were reported in 57%, 60%, and 84% of the 3-, 4-, and 5-day CIV IL-2 patients, respectively. Serious adverse events, mainly due to the requirement of hospitalization, occurred in 20% of IL-2 recipients, compared with 10% of control patients. Viral load during the course of the study was not different among the treatment groups. IL-2 therapy in cycles of 5 days resulted in an optimal increase in CD4(+) T-cell counts and is the preferred cycle length for IL-2 therapy geared toward increasing CD4(+) T-cell numbers.