A randomized controlled clinical trial for the use of immunotherapy for maintaining patients with acute myelogenous leukaemia has been conducted involving 52 patients. All patients received short pulsed maintenance chemotherapy but 30 of the patients received additional immunotherapy consisting of BCG and irradiated allogeneic myeloblastic leukaemia cells. Of the 22 patients who received chemotherapy alone, 19 died with a medium survival of 39 weeks, whereas 20 of the 30 patients who received chemotherapy plus immunotherapy died with an estimated median survival of 74 weeks. The value for the significance of the difference in death rates of patients was less than .01. The main effect of this immunotherapy was, however, due to a prolongation of survival after the patients had relapsed; the median survival for the immunotherapy patients being 23 weeks compared with only 11 weeks for the chemotherapy alone patients (p less than .005). The overall results also show that there was a slight prolongation of first remission in patients who received immunotherapy with a median duration of 44 weeks compared with 27 weeks for patients who received only chemotherapy. The possible mechanisms of action of this form of immunotherapy are discussed.