The aim of this study was to analyse the interplay between the treatment responses, as assessed by serum HIV-1 RNA levels and CD4 cell counts, virological phenotype and the development of phenotypic and genotypic resistance. A total of 47 late-stage, HIV-1-infected, antiretroviral-naive patients treated with reverse transcriptase inhibitors (zidovudine or didanosine monotherapy or alternating zidovudine and didanosine) as part of a randomized study and remaining on treatment for a minimum of 1 year were included in the study. Baseline serum HIV-1 RNA levels did not differ between the patients harbouring syncytium-inducing (SI) virus and those harbouring non-syncytium-inducing (NSI) virus (P = 0.66), despite the fact that the group of patients with SI virus had a significantly lower median CD4 cell count (P < 0.00005) and a higher proportion of patients diagnosed with AIDS at study entry (11/19 versus 6/25) than did the group with NSI virus. The patients harbouring SI virus had significantly faster clinical progression than that of the patients harbouring NSI virus (P < 0.001). The patients wit