Cell-free human immunodeficiency virus type-1 (HIV-1) was precipitated from archival serum with polyethylene glycol (PEG), and HIV-1 RNA was detected and quantified by reverse transcription and amplification in a nested polymerase chain reaction (PCR). The assay of end-point dilutions cDNA in nested PCRs allowed an estimation of the minimum RNA copies per unit volume of serum. RNA titres correlated with the classification of HIV-1 infection by CDC disease groups (30 patients). The geometric mean titres of HIV-1 serum RNA from patients grouped by disease stage gave minimum estimates of 340 and 400 virions per millilitre of serum in CDC groups II and III (n = 6 and 10, respectively) and 4,240 virions per millilitre in CDC group IV (n = 14). An overall fall in viral titre measured in this way was observed in 3 patients during zidovudine treatment. HIV-1 titres increased in a further 4 patients when therapy was interrupted, stopped, or complicated by secondary infection.