The objective of this randomized double-blind, placebo-controlled trial was to investigate the effect of combination antiretroviral therapy on plasma HIV-1 RNA as measured by HIV RNA PCR and to assess the safety and tolerability of such regimens. The trial was carried out in seven European countries, Australia and Canada and involved antiretroviral-naive patients (n = 106) with CD4 counts between 150-300 cells/mm3 (CDC group A) and 150-500 cells/mm3 (CDC group B/C). Patients were randomly assigned to zidovudine (200 mg three times daily) plus lamivudine (300 mg twice daily) or to zidovudine plus lamivudine plus loviride (100 mg three times daily) for 52 weeks. The main outcome measures were degree and duration of reduction of plasma HIV-1 RNA as measured by RNA PCR and the development of drug-related toxicities sufficiently severe to warrant dose modification, interruption or permanent discontinuation. A mild, though statistically significant difference in favour of zidovudine plus lamivudine plus loviride for log10 plasma HIV-1 RNA (P = 0.022), as compared to zidovudine plus lamivudine, was observed using area-under-the-curve minus baseline (AUCMB). An increase in CD4 cell count in the zidovudine plus lamivudine plus loviride group was observed with a median improvement of 124 cells/mm3 at week 52 compared with 70 cells/mm3 in the zidovudine plus lamivudine group (P = 0.06). Both treatment regimens were well tolerated.