The aim of the present study was to analyze the nucleolar organizer region (AgNOR) pattern of granulopoietic precursors in chronic myeloid leukemia (CML) at diagnosis and during the course of the disease. Clusters of AgNORs and isolated dots were counted separately in 24 cases of CML at diagnosis, in 19 cases during the relapse of the chronic phase after treatment, and in 16 cases of blast crisis. For comparison, 20 cases of normal bone marrow were studied. Each cell type had its own characteristic AgNOR pattern, as has been described for normal bone marrow. There was no significant difference in the number of AgNORs between cells in the peripheral blood and bone marrow. Compared with normal granulopoiesis, myeloblasts in CML at diagnosis had lower numbers of clusters, which decreased further during relapse of chronic phase and in blast crisis. Promyelocytes and myelocytes showed significantly fewer dots. The number of AgNOR clusters correlates inversely with the duration of the cell cycle. Therefore, these findings are consistent with the progressive loss in proliferative activity of immature precursors described during the course of CML. As the number of dots indicates cellular maturation, their lower number in promyelocytes and myelocytes in CML favors the concept of a discordant maturation process described in this disease. The separate counting of clusters and dots provides a useful, simple, and cheap method of describing cytokinetic changes during the course of this myeloproliferative disorder.