A multi-institutional randomized study for the evaluation of ubenimex (Bestatin) in the treatment of adult acute nonlymphocytic leukemia was performed. One hundred and ninety-five patients were registered from February 1988 to December 1990. Patients who had reached complete remission by one or two courses of remission induction chemotherapy were divided into the ubenimex group or the control group by randomization. Patients of the ubenimex group started to receive 30 mg ubenimex orally once a day when maintenance therapy began and continued as long as possible. Remission duration and survival were analyzed based on the data as of August 31, 1991. Remission duration of the ubenimex group was superior to that of the control group (generalized Wilcoxon test: p = 0.0338). Fifty percent remission duration was 508 days in the ubenimex group and 386 days in the control group. There has been no statistical difference in survival between the two groups as yet.