The effect of immunotherapy with Nocardia rubra cell-wall skeleton (N-CWS) on remission duration and survival of adults with acute myelogenous leukemia (AML) was studied in a prospective randomized controlled study. After having been induced into complete remission and having been consolidated, 73 patients were randomized either to maintenance chemotherapy or maintenance chemotherapy plus immunotherapy with N-CWS and irradiated allogeneic AML cells. Thirty-four patients in the chemotherapy group and 32 in the chemoimmunotherapy group were evaluable. Six months after the closure of the study, the immunotherapy showed a borderline beneficial effect on remission duration (P = 0.080) and on survival length (P = 0.098). When the data were analyzed at 30 months after the entry, there was a borderline significant difference in remission duration (P = 0.080) between the two groups, prolonging the 50% remission period by 110 days; but no significant difference in survival length (P = 0.314), although the 50% survival was 168 days longer in the chemoimmunotherapy group. However, there were 4 (18.2%) 5-year relapse-free survivors among 22 patients (11 in each group) who had been diagnosed more than 5 years before the time of the present analysis, and all of them belonged to the chemoimmunotherapy group (P = 0.090). Thus, immunotherapy with N-CWS and irradiated allogeneic AML cells seems to be active in the treatment of adult AML when used for maintenance therapy in combination with chemotherapy.