Human T-lymphotrophic virus type II (HTLV-II) has not yet been associated with any disease. Little is known about the proviral loads of HTLV-II in vivo and its relationship, if any, to lack of pathogenicity. We determined the HTLV-II proviral copy number in peripheral blood lymphocyte (PBL) samples from 49 HTLV-II-infected individuals, of whom 25 were coinfected with human immunodeficiency virus type 1 (HIV-1). The HTLV-II copy numbers were determined by polymerase chain reaction (PCR) amplification of end-point dilutions of PBL lysates, followed by hybridization to a 32P-labeled HTLV-II-specific probe. The proviral copy number for the 49 samples ranged from < 0.02 to 200 per 1000 PBLs; 6% had < 0.02, 16% had 0.02, 20% had 0.2, 18% had 2, 31% had 20, and 8% had 200 copies per 1000 PBLs. The distributions of HTLV-II copy numbers in the coinfected and singly infected subgroups were not significantly different (Wilcoxon rank sum, p = 0.24). In the coinfected subgroup, there was no significant correlation between the HTLV-II proviral load and the counts of CD4-positive lymphocytes or CD8-positive lymphocytes (Spearman Coefficient = 0.26, p = 0.20; = 0.091, p = 0.67, respectively). Our data demonstrate the presence of a wide range of viral loads in HTLV-II-infected individuals. The high viral loads (> or = 20 copies/1000 lymphocytes) detected in 39% of our samples suggest that the low pathogenicity of HTLV-II is not related to the presence of low viral loads in the infected subjects. Our data from the HIV-1 coinfected individuals show no apparent effect of HIV-1 on HTLV-II proviral loads.