When leukemic blood or marrow specimens from 148 adults with newly diagnosed acute nonlymphocytic leukemia (ANLL) were studied with chromosome banding techniques, 79 were found to have clonal abnormalities. Among 130 treated patients, the 53 with initially normal karyotypes had a significantly longer survival rate than the 16 in whom no normal metaphases were observed (p = 0.02). The 55 patients with both normal and abnormal metaphase cells had an intermediate survival. Once a complete remission had been attained, however, there was no significant difference in median survival between those patients with entirely normal karyotypes and those with abnormal karyotypes. Among the various FAB morphologic subsets of ANLL, the differences in complete remission rate and overall survival between the various cytogenetic subsets were greatest in acute myelogenous leukemia (AML, M1 + M2). The presence of an abnormal clone was a more important predictor of clinical outcome (p = 0.02) than the presence of normal stem cell clones. Aneuploidy alone (hyperdiploidy or hypodiploidy) was not of predictive value, indicating that the use of banding techniques to identify structural rearrangements in pseudodiploid cells was essential. Clonal chromosomal abnormalities were nonrandom and acquired, and specific abnormalities were closely associated with specific clinical-pathologic subsets of ANLL. All 13 patients with acute promyelocytic leukemia and adequate cytogenetic specimens had t(15;17); this translocation was not found in any other subset of ANLL. Six patients with AML (M2) had t(8;21) or a variant of this rearrangement. Seven patients had inv(16)(p13q22) associated with acute myelomonocytic leukemia (AMMoL, M4) and abnormal marrow eosinophils. Two patients had ins(3;3) and thrombocytosis. Four patients had a translocation involving 11q, but none of these had acute monocytic leukemia (AMoL, M5); no patient had del(11q).