Chromosome analyses were performed by a direct method on bone marrow cells of 147 patients with acute leukemia and preleukemia; in 53 chromosomally abnormal cell lines were found. Chromosome abnormalities due to structural alterations were observed in 48% of the aneuploid patients. Using the ASG banding technique, the exact identification of the abnormal chromosomes was successfully made in 22 aneuploid patients. Even though variability between patients existed in the chromosome changes; the nonrandom occurrence of some chromosome abnormalities was revealed, involving most frequently chromosomes No. 8 and No. 21. Abnormalities of chromosome No. 22 were not encountered, contrasting sharply with the frequent involvement of this chromosome in chronic myelogenous leukemia. The significance of the preferential involvement of No. 8 and No. 21 chromosomes is discussed in relation to leukemogenesis.