Lymphocytic interstitial pneumonia (LIP) is a rare form of interstitial pneumonia with infiltration of mononuclear cells in the interstitium, the pathogenesis of which is unknown. We studied six patients with LIP to investigate the immunologic characteristics of lymphocytes in the lower respiratory tract and the possible involvement of human T-cell lymphotropic virus type I (HTLV-I), which is endemic in the southwestern region of Japan. Lymphocyte surface antigen phenotyping in peripheral blood (PB) and bronchoalveolar lavage fluid (BALF) cells revealed a predominance of CD8 positive (suppressor/cytotoxic) T-lymphocytes (PB: 51.5 +/- 0.8%; BALF: 52.9 +/- 12.7%), which consisted of an increased number of CD8 positive CD11 negative (cytotoxic) T-lymphocytes (PB: 30.2 +/- 10.5%; BALF: 57.1 +/- 11.8%). The increased number of cytotoxic T-lymphocytes among patients with LIP led us to investigate the possible involvement of HTLV-I, EB virus, and HIV in patients with chronic interstitial disorders of LIP, sarcoidosis, and IPF. The seropositivity rate for HTLV-I was 83.7% (5/6) in patients with LIP, 3.1% (1/28) with IPF, 7.1% (2/28) with sarcoidosis, and 0% (0/28) in healthy volunteers. None of the patients were seropositive for either HIV or EB virus. These results suggest that HTLV-I may be involved in a direct or indirect role in the pathologic mechanisms through the host immune reaction to the antigenicity of the virus and/or the possible involvement of viral regulatory genes to disrupt the normal immune reaction of the host.