Sera from 91 adult acute nonlymphocytic leukemia patients were tested for circulating immune complexes by the C1q binding test. In 42 patients tested prior to the initiation of therapy, C1q binding activity was inversely correlated with leukemic infiltrate of the bone marrow (rs = -0.34, p less than 0.02), but it was not related to peripheral white blood cell count or presence of infection. Patients with abnormal C1q binding activities (greater than 6%) were more likely to have cytogenetic abnormalities in their bone marrow (p = 0.02); the most frequent abnormality was an extra No. 8 chromosome. The median survival of 8 patients with abnormal values was 1.5 months as compared to 7.8 months in 34 patients with normal values. However, the C1q binding test did not identify patients likely to achieve remission; and after remission was achieved, it did not identify those likely to relapse. Differences between these findings and those recently published by other investigators are discussed.