Bone-marrow chromosomes were examined with the G-banding technique in 30 patients with acute myeloid leukemia at the time of diagnosis. In 13 of the 30 patients (43%) only normal diploid bone-marrow cells were found, and no deviations from the normal banding pattern could be detected in these cells. In bone-marrow cells of 17 patients (57%), distinct chromosome abnormalities were found; in 10 of the patients only abnormal cells were observed, whereas in 7 of the patients the abnormal cells coexisted with normal diploid cells without any visible chromosome banding abnormality. The results of the detailed analysis of the karyotypic aberrations demonstrated that when chromosome aberrations occurred they were clearly non-random. All patients except two displayed trisomy 8,9 or 21 or monosomy 7. Analysis of cases of acute leukemia from other laboratories indicated that the same consistent pattern of chromosome involvement prevailed in them.