Our simple rapid autoradiography for 3H-thymidine (3H-TdR) labeling index (LI) of leukemic cells was introduced. Thirty-nine adult patients with acute nonlymphocytic leukemia were treated under a new protocol using 3H-TdR LI as a parameter and inserting a trial of partial synchronization by Ara-C. The success rate of synchronization by Ara-C (rise of the LI up to 10%) was 62.5% out of 32 courses. By our regimen the complete remission (CR) rate of 71.4% in 35 first treated cases, that of 44.0% in 25 courses of recurrent cases, and a median survival of 12 months were achieved. A survey of literature revealed that our results were favorably compared with others. Our new protocol is advantageous in permitting one to precisely know the proper time for chemotherapy and thus avoiding unnecessary side effects.