Sarcoidosis is a granulomatous disorder of unknown etiology. Accumulated data suggest that one or several exogenous or altered self antigens participate in producing pathophysiological change in sarcoidosis. Recently, analysis of retroviruses such as HTLV-1 and HIV-1 revealed that these viruses would produce autoimmune disease like symptoms including interstitial lung disease like pulmonary manifestation. We hypothesized novel type retrovirus or retrovirus related antigens might be a putative pathogen for sarcoidosis. Syncytial cell formation or cytopathic effect was observed in 6 of 24 patients (25%) after coculture of sarcoid BALF cells with U937 cells. Five of 18 culture supernatant showed moderate reverse transcriptase activity. Expression of clone 4-1 env protein, one of the endogenous retroviral elements, was also observed in alveolar macrophages of sarcoidosis. These data encourages the further investigation of retrovirus as the pathogen of sarcoidosis.