Thirty-nine adults with acute nonlymphoblastic leukemia (ANL) in complete remission (CR) for six months were considered for intensification therapy. All patients had received a high dose chemotherapeutic remission-induction regimen consisting of daunorubicin, cytosine arabinoside, 6-thioguanine, prednisone, and vincristine, followed by one consolidation course of the same drugs at reduced doses and then monthly maintenance course of low-dose chemotherapy. The intensification therapy consisted of the same intensive chemotherapy regimen utilized for remission induction in place of the sixth and 12th monthly courses of postremission induction chemotherapy. Twenty-three of the 39 patients received intensification therapy, whereas 16 patients refused or were not offered such therapy for medical reasons and, therefore, received only monthly therapy. The median remission duration of the 23 patients who received intensification therapy was 157 weeks, and 9 remain in continuous first remission at 176-285 weeks. The remission duration of the 16 patients who did not receive intensification therapy was 73 weeks (P = 0.03). Kaplan-Meier estimates of remission duration and survival from time of remission computed by including the patients who either relapsed or received bone marrow transplantation before the time of intensification (6 months) revealed a median remission duration and survival of 100 and 172 weeks for patients receiving intensification compared to 37 and 72 weeks, respectively, for patients not receiving intensification therapy. Since results in patients not receiving intensification therapy are consistent with our previous results in patients receiving the same induction and consolidation regimens without intensification, this nonrandomized study suggests that intensification therapy prolongs remission duration and survival in adults with ANL.