In Japan a number of reported cases of diffuse panbronchiolitis (DPB) have been associated with human T lymphotropic virus type I (HTLV-I) infection. In this study the hypothesis that HTLV-I proviral DNA may be prevalent in DPB was examined using polymerase chain reaction (PCR) for the region of env or the two-step PCR for the pX region of this virus. The presence of HTLV-I proviral DNA was studied in the peripheral blood mononuclear cells (PBMC) obtained from 10 patients with DPB. The presence of proviral DNA in PBMC in 12 patients with chronic obstructive pulmonary disease (COPD), eight patients with idiopathic interstitial pneumonia (IIP), four patients disease were also studied as relevant controls. The lung tissue obtained from 11 patients with DPB, 12 patients with diffuse aspiration bronchiolitis (DAB) at autopsy, and the surgical lung samples obtained from 12 patients with bronchogenic cancer were also studied. Peripheral blood mononuclear cells obtained from one DPB patient and one bronchogenic carcinoma patient were positive for the HTLV-I pX region. The presence of the pX region was also found in the lung tissue of three DPB patients (27.3%) and one DAB patient (8.3%). None of other subjects were positive for HTLV-I proviral DNA, In conclusion, HTLV-I is not the causative virus in the pathogenesis of COPD, IIP, bronchiectasis and bronchogenic carcinoma. There is a likelihood that HTLV-I infection is associated with some cases of DPB; however this association needs further verification.