Twin girls, genetically identical, probably experienced different leukemogenic events and presented with acute lymphocytic leukemia 6 years apart. Their clinical presentations were similar, but they received significantly different therapy. The first twin died 34 months after diagnosis following multiple remissions and relapses, having received single-drug maintenance. The second twin remains free of apparent disease 60 months after diagnosis, following vincristine and prednisone induction, 6-mercaptopurine maintenance, methotrexate and prednisone reinforcement, and central nervous system treatment of occult disease. Their dissimilar clinical courses may have been due to different leukemogenic events and/or markedly different therapeutic programs.