Results of the bone marrow transplantation (BMT) for 120 cases of leukemia, which were done in nine institutes in Nagoya (Nagoya Bone Marrow Transplantation Group) last ten years, were analyzed to determine the factors associated with an increased risk of developing interstitial pneumonia (IP). IP developed 49 out of 120 patients (49.8%) and case fatality rate was 63.3%. The median time from transplantation to onset of IP was 81 days (range 13-575 days), in 30 out of 49 cases (61.2%), this complication developed within 100 days after transplantation. Of the 49 patients who developed IP, cytomegalovirus (CMV) infection was associated in 18 cases (36.7%), no cases of P. carinii infection was detected. Five factors were associated with an increased risk for developing IP, (1) older age (greater than or equal to 47.0%: less than 10 y. 10.0%) (p less than 0.01) (2) disease stage at BMT (non-remission 76.2%: remission 32.5%) (p less than 0.01) (3) presence of acute GVHD ((+) 52.5% (-) 28.8%) (p less than 0.05) (4) onset day after BMT (less than or equal to 100 days 61.2%: greater than 100 d. 38.8%) (p less than 0.01) (5) sex matching between donor and patient (sex match 28.8%: sex mismatch 57.1%) (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)