Cytogenetic studies were performed at diagnosis and in relapse in 45 patients with acute nonlymphocytic leukemia. While 23 patients did not exhibit clonal chromosome changes, the other half had changes of different types: apparent acquisition of clonal abnormalities in 4 cases normal at diagnosis; appearance of additional chromosome changes either in the same clone or as mosaic in 9 cases; selection of a minor subclone previously observed at diagnosis in 7 cases; disappearance of the abnormal clone replaced by a cytogenetically unrelated karyotype in 2 cases. Some non clonal anomalies similar to those observed in secondary leukemias were also detected.