The effect of immunotherapy with a protein-bound polysaccharide preparation termed PSK on remission duration and survival of adults with acute nonlymphocytic leukemia (ANLL) was studied in a prospective randomized cooperative trial. After having achieved complete remission and receiving a consolidation therapy, 73 patients were randomized either to maintenance chemotherapy or to maintenance chemotherapy plus immunotherapy with PSK. Ultimately 36 patients in the chemotherapy group and 31 in the chemoimmunotherapy group were evaluable. Six months after the last entry, immunotherapy with PSK showed a borderline beneficial effect on remission duration (P = 0.089) and on duration of survival (P = 0.062). When the data were analyzed 12, 18, and 24 months after the last entry there were no significant differences in duration of remission and survival between the two groups. However, analysis of the data of patients who had maintained complete remission for more than 270 days revealed that immunotherapy had a suggestive beneficial effect (P = 0.105), prolonging the 50% remission period by 418 days (885 vs 467 days). Thus, immunotherapy with PSK seems to be active in the treatment of adult ANLL when used for maintenance therapy in combination with chemotherapy, especially in patients with a good prognosis.