The development of human T-cell leukemia type 1 (HTLV-1) diseases are related to an increase in the proviral copy number (VCN) in peripheral blood mononuclear cells (PBMCs). Twenty symptomless anti-HTLV-1-positive blood donors, as well as four symptomatic individuals, all from the French West Indies, were studied. The VCN in PBMCs was determined by quantitative PCR. The VCN values for asymptomatic HTLV-1 carriers (range of less than 100 to approximately 9,500/micrograms of DNA) was nearly always less than the values for symptomatic carriers (range of approximately 5,500 to approximately 29,000/micrograms of DNA). Consequently, the proportion of HTLV-1-infected PBMCs in symptomless and in symptomatic individuals ranged from less than 1/1,500 to approximately 1/16 and approximately 1/27 to approximately 1/5, respectively. No correlation could be found between VCN and age or sex, suggesting the importance of factors other than age and sex as influences on the VCN number.