The proviral DNA of human T-lymphotropic virus type I (HTLV-I) was examined in 8 patients with adult T-cell leukemia/lymphoma (ATL/L) by Southern blot analysis. Judging from the number of bands containing viral DNA sequences in tumor cells, one and three copies of proviral genome were detected in 7 and 1 cases, respectively. The proviral genome in 1 case contained variant PstI sites and a single defective provirus which lost 5' LTR, gag and a part of pol genes was found in 2 cases. However, the pX region was detected in both of 2 latter cases by using the polymerase chain reaction. The results indicate that not all tumor cells from ATL/L yield infectious viruses and also support that the product of pX region may contribute to the tumorigenesis of HTLV-I.