An 82-year-old woman with stage I chronic lymphocytic leukemia presented with systemic symptoms, minimal adenopathy, hepatosplenomegaly, and anemia five years after the initial diagnosis was made and while receiving no therapy. Her white blood cell count was 231,000/mm3 with an absolute neutrophil count of 164,360/mm3 and lymphocyte count of 43,890/mm3. Peripheral blood smear inspection revealed both increased mature lymphocytes and myeloid cells at all stages of maturation. Flow cytometric analysis of forward- and right-angle light scatters demonstrated the presence of two populations of cells, one lymphoid, bearing predominantly lambda light chain surface immunoglobulin and showing phenotypic characteristics of B cell chronic lymphocytic leukemia (HLA-DR-positive, BL-1-positive, BL-2-positive, BL-7-positive, Leu-1-positive, Leu-10-positive, BL-5-negative, BL-6-negative, and OKM1-negative), and another granulocytic population expressing phenotypic features compatible with myeloid lineage (HLA-DR-negative, Leu-1-negative, BL-1-negative, BL-2-negative, BL-7-negative, Leu-10-negative, BL-5-positive, BL-6-negative, OKM1-positive, and surface immunoglobulin-negative). All of the peripheral blood cell metaphases were Philadelphia chromosome-positive after 24 hours of culture, confirming the diagnosis of chronic myelocytic leukemia, whereas all of the Epstein-Barr virus-treated B lymphocyte metaphases showed a normal karyotype after two weeks of culture. In this patient, analysis of surface antigens and immunoglobulin fractions by flow cytometry proved to be useful in recognizing concomitantly expressed leukemic lineages. This approach allows the increasing recognition of the heterogeneity of leukemic populations.