Bone marrow chromosomes obtained from 50 of 55 consecutive adult patients with acute nonlymphocytic leukemia were analyzed with quinacrine fluorescence. Twenty-five patients showed a normal karyotype and 25 an abnormal karyotype on the initial samples available for analysis. Among the 25 patients with abnormalities, the marrow cells contained 48 chromosomes in one case, 47 in two, 46 in ten, 45 in nine, 43 in two, and 42 chromosomes in one case. Seven of the ten patients with 46 chromosomes had abnormalities, primarily balanced translocations, that were not detected with the standard Giemsa stains. The analysis of all of the data available revealed the presence of nonrandom chromosome changes such as the addition of No. 8, the loss of No. 7, and a gain or loss of one No. 21. the most frequent structural rearrangement was the translocation between the long arm of No. 8 and No. 21, which may also be associated with the loss of a sex chromosome. Chromosomal abnormalities decreased or disappeared during remission; the same abnormality recurred in relapse. Chemotherapy did not appear to produce a stable clone of aberrant cells. Evolution of the karyotype occurred in eight patients, in five of whom an additional No. 8 was observed. This pattern of chromosomal evolution in patients with acute leukemia was very similar to that observed in patients with chronic myelogenous leukemia in the blast phase.