Data from 176 patients with acute leukaemia given allogeneic marrow transplants between Jan. 1, 1977, and Dec. 31, 1980, and reported to the International Bone Marrow Transplant Registry were analysed retrospectively for prognostic factors associated with the development of interstitial pneumonitis (IPn). The overall incidence of IPn was 20% (36/176), and the disease was fatal in 21 of the 36 cases (58%). Three of more than thirty prognostic factors studied seemed to be associated with a low risk of IPn--pretransplant total body irradiation at a dose-rate less than or equal to 5.7 cGy/min, when compared with higher dose-rates (p less than 0.001); post-transplant immunosuppression with cyclosporin-A, when compared with methotrexate (p less than 0.0003); and transplantation of cells from female donors into female recipients (p less than 0.0005). The low dose-rate of total body irradiation and the use of cyclosporin-A were considered as independent variables, but were confounded to the extent that it was not possible to determine if one or both factors were associated with a decreased incidence of IPn.