The purpose of this study is to see if immunotherapy with C. parvum and prevention of central nervous system relapse with intrathecal methotrexate can prolong duration of complete remission and survival as well as avoid central nervous system relapse. For induction, three weekly I.V. injections of vincristine and Daunorubicin were given with daily prednisone orally, followed by 5-day courses of Cytosine Arabinoside and 6-mercaptopurine. The patients were randomized to chemotherapy or chemoimmunotherapy. Maintenance consisted of vincristine, Daunorubicin, and prednisone one week every odd month, and a 5-day course of Cytosine Arabinoside and 6-mercaptopurine every even month. Every week, the chemoimmunotherapy group also received, without chemotherapy, one injection of C. parvum 4 mg, subcutaneously. All patients received five weekly injections of intrathecal methotrexate 13 mg/m2 right after complete remission was achieved. Out of 181 evaluable cases, 80 (44%) achieved complete remission, 45 were randomized to chemotherapy, and 35 to chemoimmunotherapy. In the chemoimmunotherapy group 32/35 relapsed, and in the chemotherapy group 36/45. Median duration of complete remission and survival were: for group chemotherapy, 8 and 15 months; for group chemoimmunotherapy, 5 and 10 months. This difference is not significant. Intrathecal methotrexate was given to all patients. Six patients (7%) had central nervous system leukemia at the time of the first injection. None had central nervous system relapse after prevention with intrathecal methotrexate. This method seems useful in preventing central nervous system relapse in patients with acute myeloblastic leukemia, but does not seem to prolong complete remission.