An escalating-dose trial of idarubicin, used weekly for 3 doses in combination with vincristine, prednisone, and L-asparaginase (VPLI), to reinduce remission of childhood ALL at first bone marrow relapse was conducted by the Childrens Cancer Study Group (CCSG). The maximum tolerated dose (MTD) of idarubicin, used in the manner, was determined to be 12.5 mg/m2/dose. Twelve of 16 (75%) evaluable patients in first marrow relapse of ALL treated at a dose of 10 or 12.5 mg/m2 entered a second complete remission, compared to 41 of 69 evaluable patients (59%) treated in a comparable way with daunorubicin (30 mg/m2) (VPLD). Prolonged myelosuppression was observed in both groups, but the frequency of documented bacterial sepsis and the duration of required hospitalization were greater among patients treated with idarubicin. No additional toxicity, specifically attributable to idarubicin, was observed at these doses.