The incidence of circulating granulocyte-macrophage colony-forming cells (CFU-c) was determined in 60 patients in different stages of chronic myelocytic leukemia (CML). Like others, we found uniformly increased circulating CFU-c during the uncontrolled chronic stage, decreasing to values indistinguishable from those of healthy controls during remission. Unlike some investigators who described grossly deficient colony formation during the blastic stage of CML, we found normal to greatly increased colony formation in the accelerated-resistant and blastic stages. The fact that laboratories using somewhat different culture techniques obtain similar results with specimens from the chronic stage of CML but divergent results with specimens from terminal stage disease suggests that CFU-c from blastic disease have more fastidious growth requirements than do those from chronic stage disease or from normal subjects. In contrast to the correlation between CFU-c and disease status in the chronic stage of CML, CFU-c incidence in the accelerated-resistant and blastic stages of the disease did not correlate with white blood cell count, percentage of immature cells, clinical status, or survival. There was no correlation between the percentage of myeloblasts and promyelocytes in circulating blood and the incidence of CFU-c in any stage of CML, which suggests that no direct relationship exists between clonogenic units and the number of identifiable proliferating cells.