A total of 144 human immunodeficiency virus (HIV)-infected patients (mean CD4 cell count, 367 cells/mm3) were included in a double-blind placebo-controlled trial testing the efficacy on surrogate markers of HIV progression of the combination didanosine (2',3'-dideoxyinosine or DDI) plus stavudine (2',3'-didehydro-2',3'-dideoxythymidine or D4T) with or without hydroxyurea. The primary end point was a reduction of HIV RNA levels to below 200 copies/ml after 12 weeks of treatment. The results showed that the triple combination was associated with a more profound decrease in HIV RNA with an increased proportion of patients with viraemia < 200 copies/ml. This effect persisted for the majority of the patients after a 48 week follow-up. In contrast, the increase in CD4 cell counts was less in patients treated with hydroxyurea because of lymphopenia, and adverse events were more frequent in hydroxyurea-treated patients. In conclusion, the addition of hydroxyurea consistently improved the antiviral activity of the didanosine/stavudine combination over a 48 week follow-up. Increased toxicity and decreased effect on CD4 cell counts might inspire caution.