Preliminary results of an in vitro study of the proliferation characteristics and karyotypes in the cells of a series of 20 patients with ANLL are presented. The leukemic cells exhibited in vitro proliferation patterns of essentially three types, all of which were very different from the proliferation pattern of normal bone marrow. Correlation with the karyotypic findings did not confirm that the cell cycle may be longer in aneuploid cells. To the contrary, a substantial part of the ANLL presented a karyotype that was normal by routine banding technique standards, in spite of the presence of a strikingly abnormal proliferation pattern of the dividing cells. These observations are important for a better understanding of the significance of chromosome anomalies in the development of leukemia, as well as for the clinician who, in the absence of chromosome markers, may use the proliferation pattern in the monitoring of leukemic patients.