We report on a 69-year-old man who developed Ph-positive CML 6 years after the onset of B-cell CLL. When CML was diagnosed, both malignant cell populations were detected in bone marrow and peripheral blood. Peripheral leukocytes were fractionated by Ficoll-Hypaque density gradient, and cytogenetic and molecular studies were performed on mononuclear cell and granulocyte-enriched populations. Mononuclear cells were stimulated with either PHA or PWM. In PHA-treated cultures 76% of the metaphases were Ph-negative, while after PWM stimulation 87% were Ph-positive. A bcr rearrangement was observed in DNA from the granulocyte-enriched fraction, but not in mononuclear cells. On the contrary the IgH locus resulted in monoclonally rearranged DNA, only in peripheral blood mononuclear cells. These results indicate that the two neoplastic populations originated independently.