Human T cell leukemia virus type-I (HTLV-I)-integrated synovial cell clones (SCCs) were established from four HTLV-I-infected patients with chronic proliferative arthropathy. Extracted DNAs of these SCCs were examined for the existence of HTLV-I proviral DNA. Electron microscopic analysis and immunohistochemical staining were also performed to examine the nature of the HTLV-I integrated SCCs. SCCs derived from all four patients consisted of either HTLV-I integrated or nonintegrated cells. HTLV-I integrated SCCs also produced viral messenger RNA which was detected by reverse transcriptase-polymerase chain reaction as well as viral protein which was detected by immunohistochemical staining. Phenotypically, these SCCs containing HTLV-I proviral DNA were shown to be all cell types as observed by electron microscopic analysis and immunohistochemical analysis. Moreover, expression of HTLV-I glycoprotein was greater in SCCs with a monocyte marker. These findings suggest that SCCs harboring HTLV-I have an important role in synovial hyperplasia.