Chromosome analysis on clinical leukemia material was done by means of flow cytometry (flow karyotyping) to investigate the applicability of this technique in the detection of leukemia-associated abnormalities. Flow karyotyping was performed on blood or bone marrow samples from eight patients with chronic myelocytic leukemia (CML) after a culture period of four days and arresting the cells in metaphase during the last 16 hours. Discontinuous density gradient centrifugation proved to be essential in removing debris and dead cells from the cell suspensions. By this procedure the mitotic index increase ranged from 2 to 80 times initial values. Chromosomes were isolated and stained with two base pair-specific fluorochromes, ie, chromomycin A3 and Hoechst 33258, and run through a specially designed dual-laser beam flow cytometer. Generally, 20,000 chromosomes or more were measured. The data were computer stored in list mode. Besides the clear detection of the specific Philadelphia chromosome, trisomies and other additional chromosomal aberrations [like an i(17q)] were visualized. Quantitative analysis revealed the percentage of subclones containing a certain chromosomal anomaly. Conventional cytogenetic analysis confirmed these findings. In seven of eight cases, CML could be diagnosed on the basis of the presence of a Philadelphia chromosome in the flow karyogram. In one of these seven, the conventional cytogenetic analysis was unknown at that time. The remaining six all matched the standard cytogenetics. The one failure out of eight could be attributed to the specific stimulating conditions in the culture. Although it is impossible by this technique to determine the position of the breakpoint, the involved chromosomes in the translocation event could be identified. In some cases, low percentages of aberrations could not be detected. This study shows that CML can be diagnosed on the basis of flow karyotypic results. Additional chromosomal aberrations can be detected provided that changes in the amount of DNA per chromosome have occurred. Exact quantification of the composition of subclones in the case of mosaicism appear difficult.