Two different Total Body Irradiation (TBI) regimens were employed (1981 to July 1983) in Genoa in the conditioning program for the allogeneic Bone Marrow Transplantation (BMT) of 22 patients suffering from Acute Lymphoblastic Leukemia (ALL) in remission (7 patients in 1st remission, and 15 in 2nd remission). All patients were treated with Cyclophosphamide -60 mg/kg administered for two consecutive days (day -7 and -6)--and subsequently underwent fractionated TBI (days -3, -2, -1), that is, our conventional TBI regimen: 3.3 Gy/day per 3 days (total dose: 9.9 Gy). From August 1983 through 1988, 33 patients (14 in 1st remission and 19 in 2nd remission) were given 2 Gy twice a day, 6 hours apart, for 3 consecutive days (total dose: 12 Gy). Cyclosporine A was used for GvHD prophylaxis. At 58 months, out of the total figure of ALL patients in 2nd remission, 19% of those treated with 9.9 Gy/3 fr/3 days (fractionated TBI) is likely to be in remission, versus 65% of the cases treated with 12 Gy/6 fr/3 days (p less than 0.01) (hyperfractionated TBI); the actuarial overall survival is 23% after fractionated vs 60% after hyperfractionated TBI (p = 0.05). The incidence of idiopathic interstitial pneumonitis was very low (3.6%). Thus, we conclude that, in ALL patients in second remission, hyperfractionated TBI (12 Gy/6 fr/3 days) yields better results than fractionated TBI (9.9 Gy/3 fr/3 days), with lower relapse rate (33% vs 83%) and higher survival.