The incidence of interstitial pneumonitis (IP) was reviewed in 70 consecutive patients who received autologous marrow transplants for hematologic malignancies. All patients were treated with total-body irradiation (TBI), with or without other chemotherapeutic agents. Seven patients (10%) developed IP, 3 were due to cytomegalovirus, 1 due to Pneumomcystis carinii, and 3 of unknown cause (idiopathic). Risk factors for developing IP were increasing age and a prior history of irradiation to the chest. The use of methotrexate posttransplant did not increase the incidence of IP.