Thirty-two adults with acute myeloid leukaemia (AML) were randomized to receive, from the time of diagnosis, either chemotherapy alone (C group) or chemotherapy plus Bacille Calmette-Guérin vaccine (BCG) (C+I group). After remission induction and consolidation, chemotherapy was stopped in both groups but BCG was continued in the C+I group. The overall survival of the C+I group was significantly increased (P less than 0.05). There was no significant increase in the duration of first remission in the C+I group (0.05 less than P less than 0.1) nor in the time from first relapse to death (0.05 less than P less than 0.1). There was no significant difference in the incidence of first or second remissions, and the time taken to enter remission did not differ significantly between the two groups. Comparison with the results of other trials suggests that the use of maintenance chemotherapy in addition to immunotherapy produces longer remissions. Five patients in the C group developed leukaemic central-nervous-system (CSN) involvement, in comparison with none in the C+I group. CNS relapse did not produce a significant decrease in remission length (P greater than 0.1) but reduction in survival after CNS relapse was highly significant (P = 0.001). These results suggest that administration of BCG from an early stage in the treatment of AML may protect the CNS against leukaemic infiltration and therefore serve as a simple, innocuous form of CNS prophylaxis.