A cooperative randomized controlled study of immunotherapy with bestatin for ANLL in adults was conducted at 16 institutions. After complete remissions were induced and consolidated with combination chemotherapy, patients were randomly allocated into two groups, one with maintenance chemotherapy alone (C) and the other with the same maintenance chemotherapy plus bestatin (B). Out of 115 patients entered in this study, 101 (B): 48, C: 53) were eligible. The 50% remission duration was 20.4 months for B compared with 11.1 months for C, and comparable 50% survivals were 33.3 and 18.9 months respectively. B had significantly longer remission duration and survival time than C. Remission duration and survival time in patients under 49 years of age were not significantly different in both groups. But in patients over 50 years of age B had significantly longer remission duration and survival time than C. Side effects of bestatin were mild and transitional. These data suggest the usefulness of bestatin for the treatment of adult acute non-lymphocytic leukemia.