A 34 year-old male pigeon breeder developed a sudden fever, cough, and dyspnea with diffuse nodular shadows on the chest X-ray film and computer tomogram. His symptoms and diffuse nodular shadows on the chest X-ray film improved and gallium (Ga) scintigram showed no accumulation of 67Ga-citrate in his lungs when examined after corticosteroid therapy. Serum precipitin against pigeon serum and pigeon dropping extracts could be detected. In his bronchoalveolar lavage (BAL) fluid, total cell count, mainly lymphocytes, were increased. OKT8(+)-HLA-DR+T cells were dominant in the lymphocyte subpopulation of the BAL fluid. Pathological examination of transbronchial lung biopsy specimen showed interstitial pneumonia. Blastogenic response of peripheral blood mononuclear cells when cultured with pigeon serum could not be observed, which suggested suppressive effects of lymphocyte blastogenesis by corticosteroid therapy. An inhalation provocation test with pigeon serum was performed. The patient developed fever, leucocytosis, hypoxemia and decreased vital capacity after the inhalation provocation test. Consequently, the case was diagnosed as a pigeon breeder's lung.