Aberrations of chromosome 6 were observed in 11 of 193 cases of chronic lymphocytic leukemia diagnosed January 1, 1984-November 1, 1988 and investigated cytogenetically within 30 days after diagnosis. The 6p was rearranged in 5 cases: 4 balanced and 1 unbalanced translocation. The 6q was involved in 6 cases: 4 deletions and 2 balanced translocations. Three of the del(6q) may be identical: del(6)(q13q27). In two cases there were no additional aberrations. Aberrations of chromosome 6 correlated significantly with an advanced clinical stage, diffuse pattern of bone marrow infiltration, and increased SmlgM-fluorescence intensity. All these factors are associated with poor prognosis. Although the number of cases with 6q aberrations is still too small and the observation period too short to show significant influence on survival, the presence of 6q aberrations at diagnosis may prove useful in delineating a subtype of chronic lymphocytic leukemia with poor prognosis.