We studied the relation between the clinical course and the presence of circulating immune complexes at diagnosis and/or during complete remission in 186 patients with acute myeloid leukemia. Patients with immune complexes at diagnosis had significantly fewer complete remissions (32 vs. 94 per cent), remissions of shorter duration (median, 4.3 vs. 15.0 months), and shorter survival times (median, 1.8 vs. 22.3 months) than patients without such complexes (all comparisons, P less than 0.01). All patients with immune complexes during the first two months of remission remained in remission for less than six months, whereas only 11 per cent of patients without complexes within this period had such early relapse. Of 23 patients who relapsed after long remissions, 18 (78 per cent) had immune complexes that preceded hematologic evidence of relapse by three weeks to six months (median, 3.7 months). These findings suggest that circulating immune complexes may reflect an important aspect of the pathophysiology of acute myeloid leukemia, and that measurement of these complexes can provide useful prognostic information at diagnosis and during remission.