The objective of this study was to determine the efficacy and safety of adding zidovudine to continuous treatment with lamivudine in symptomatic human immunodeficiency virus type 1 (HIV-1)-infected patients. Forty patients were monitored throughout lamivudine monotherapy and subsequent combination therapy with lamivudine and zidovudine, which was initiated because of disease progression, declining CD4 cell counts or prolonged use of lamivudine. Eleven of these patients had been treated with zidovudine before the start of the study. The median CD4 cell count at the start of lamivudine monotherapy was 200 x 10(6) cells/l. After a median interval of 69 weeks (range 23-102 weeks), the median CD4 cell count had dropped to 110 x 10(6) cells/l. Initial improvements in all laboratory markers for antiretroviral efficacy were observed after the addition of zidovudine. The median CD4 cell count remained 18% above baseline after 48 weeks of treatment with lamivudine and zidovudine, however plasma HTV-1 RNA load and CD4 cell counts returned towards baseline during prolonged treatment in most patients. The combination was well tolerated, although anaemia was observed in nine patients. Repeated measures analysis of variance suggested a superior effect of lamivudine monotherapy in patients who had previously used zidovudine. In conclusion, zidovudine was found to be effective in patients who have been treated with lamivudine. The study stresses the need to further define the mechanisms underlying this prolonged antiviral effect.