To elucidate whether or not human T-lymphotropic virus type I (HTLV-I) infected cells exist in the lungs of HTLV-I carriers, we examined bronchoalveolar lavage (BAL) cells for the presence of HTLV-I proviral DNA by the polymerase chain reaction (PCR) method. The cells were harvested from HTLV-I seropositive individuals with or without various respiratory diseases. The HTLV-I pX region was detected from separated BAL cells by dot blot hybridization after PCR in all 14 HTLV-I seropositive individuals tested. Four out of these 14 individuals were asymptomatic HTLV-I carriers, while the other ten had symptoms, including those of bacterial pneumonia, chronic respiratory tract infection, interstitial pneumonia, atypical mycobacterial infection, bronchitis, mediastinal lymphadenitis and bronchial asthma. HTLV-I proviral DAN was detected in BAL cells from patients with a normal proportion of lavage lymphocytes and a normal CD4/CD8 ratio. These findings suggest that HTLV-I infected cells may commonly exist in the lower respiratory tract and alveolar space without producing characteristic symptoms/signs, and that any causal relation to the pulmonary lesion must therefore be carefully elucidated, in HTLV-I carriers.