Studies on N-acetoxy-2-acetylaminofluorene (NA-AAF) induced unscheduled DNA synthesis (UDS) of granulopoietic cells were performed in patients with chronic myeloid leukemia (CML). Sequential studies were carried out in some patients. Both biochemical and autoradiographic methods demonstrated that [3H]dT was incorporated into nonreplicating DNA of immature granulopoietic cells after NA-AAF damage and the 2 methods significantly correlated to each other (r = 0.63, n = 19). NA-AAF induced DNA synthesis was lower for myeloblasts than promyelocytes and myelocytes. Biochemically determined NA-AAF induced UDS was higher for immature granulopoietic cells in blood than in marrow. Sequential studies on granulopoietic blood cells suggested that phases of accelerated leukocytosis in CML can be preceded by increases of NA-AAF induced UDS. Whether increases in NA-AAF induced UDS relates to an amplification of repair enzymatic capacity closely correlated to the cellular replication capacity, or whether it reflects an increased sensitivity to DNA damage induction and the consequences thereof, was not resolved in this study. Nevertheless these results are consistent with the hypothesis that increases in NA-AAF induced UDS signal the evolution during chronic phase CML of cell populations of increasing malignancy which escape growth control.