Interstitial pneumonitis and biopsy-proven pulmonary fibrosis developed in a 35-year-old man with acute myeloblastic leukemia 4 months after conditioning with total body irradiation, etoposide and cyclophosphamide and allogeneic marrow transplantation from an HLA-identical sister. The patient had no evidence of graft-versus-host disease. Under treatment with cyclosporine and prednisone the patient became asymptomatic and radiographic changes of the chest normalized. Following discontinuation of immunosuppressive treatment the patient again became dyspneic, and chest radiographs showed bilateral diffuse interstitial infiltrates. Concurrently there was a rise in serum transaminases. Treatment with prednisone again resulted in resolution of all symptoms and normalization of radiographic and hepatic function abnormalities. This case indicates that late onset interstitial pneumonitis may respond to immunosuppressive therapy. Conceivably, such pulmonary disease may represent the first or only manifestation of chronic graft-versus-host disease.