The development of mutations conferring drug resistance was investigated in 49 antiretroviral-naive asymptomatic HIV-1 subjects with CD4+ cell counts of 250-500/mm3 given intermittent (6-week courses, 6 weeks apart) or continuous treatment with zidovudine (AZT) plus zalcitabine (ddC) over 54 weeks. The concentration of human immunodeficiency virus type 1 RNA in the plasma and the CD4 cell counts were measured every 6 weeks. The rate of decrease of HIV-1 RNA concentration in plasma after a 6-week course of AZT + ddC was similar for each treatment cycle (approximately 1-log reduction). The plasma HIV-1 RNA concentration returned to its initial level at each treatment interruption. The mean CD4 cell counts after 54 weeks in the two treatment groups were similar. Genotype analysis by sequencing the reverse transcriptase coding region from plasma viral RNA on treatment showed a lower frequency of AZT resistance mutations after 54 weeks in patients given intermittent treatment (18%) than in those treated continuously (79 %, P < 0.001). No mutations conferring ddC resistance or multidideoxynucleoside resistance were observed in either group. These findings may have clinical implications for long-term treatment strategies.