B-cell chronic lymphocytic leukemia (B-CLL) is a slowly progressive disease resulting from the clonal expansion of mature B lymphocytes. The most frequent chromosomal abnormality is trisomy 12. Recently more aggressive therapeutic approaches using myeloablative therapy and autologous stem-cell support have been developed. Phase I/II studies have resulted in molecular remission and prolonged survival. One cause of relapse may be tumor-cell contamination of the transplant. We asked whether immunophenotypically identified hematopoietic progenitor cells are part of the malignant cell population in B-CLL. In a patient with trisomy 12, two subpopulations of hematopoietic progenitor cells, CD34+/CD38+ and CD34+/CD38- cells, were isolated by fluorescence-activated-cell-sorting; the sort purity was 98%. Trisomy 12 was detected in 13% of CD34+/38+ cells and in 34% of CD34+/38- cells. These data suggest that CD34+ cells are involved in the malignant clone of patients with B-CLL. The results are of significance for future strategies using autologous stem-cell transfusion.