Cytogenetic studies were carried out on 103 unselected patients with acute myelogenous leukemia (AML) at the Clinical Center of the National Institutes of Health. Seventy-three patients (70.9%) had a normal karyotype and 30 patients (29.1%) were aneuploid. No unique chromosomal abnormalities were found in patients with aneuploidy; however, there was a significantly higher incidence of G group involvement. Four cases had a history of radiation exposure; three of these four patients had a normal karyotype and one had one Ph1 chromosome in her bone marrow cells. Another patient with no history of radiation also had one Ph1 chromosome in his marrow cells. Reclassification of AML patients with Ph1 chromosomes as a rare entity of blast crisis in chronic myelogenous leukemia (CML) rather than as AML’s is proposed. Two patients exhibited the 45 chromosome syndrome before the diagnosis of AML was made. The normal and aneuploid groups had about the same median survival time and same median date from diagnosis to chromosome study; however, none of the patients in the aneuploid group lived longer than 26 months after the date of diagnosis, while seven patients (nearly 10%) in the normal group did so, one living over 112 months.