Deletions of the long arm of chromosome 6 have been described in acute and chronic lymphocytic leukemia (ALL and CLL) and prolymphocytic leukemia (PLL), and have been associated with t(14;18)(q32;q21) in non-Hodgkin's lymphoma (NHL). Of 55 cases of small lymphocytic (sm lym) NHL, deletions of 6(q21q23) were the most common recurring cytogenetic abnormality. Among 14 sm lym NHL with del(6)(q21q23), this abnormality occurred as a solitary change in 3 cases. Each of these 3 cases, and 5 additional cases with del(6q) and other abnormalities, showed atypical larger forms with the morphologic appearance of prolymphocytes or paraimmunoblasts in the peripheral blood. In comparison, of the 11 cases without del(6q) and circulating abnormal cells, prolymphocytoid forms were observed in 4 cases (P < .001). Of the 31 sm lym without del(6q), trisomies of chromosomes 3, 12, or 18, or t(11;14)(q13;q32) occurred in greater than 10% of cases. Proliferation centers or infiltration by larger forms were observed in similar proportions of tissue sections derived from sm lym NHL with or without del(6q). The presence of the larger forms in the peripheral blood did not have an adverse prognostic impact on the survival of the del(6q) cohort, who experienced a median survival in excess of 6 years. All 14 cases of del(6q) sm lym NHL were characterized by a mature B-cell phenotype. Expression of CD11c, a feature of a CLL/PLL variant previously described, was not detected in 9 cases analyzed. In 5 cases of del(6q) sm lym NHL, no circulating abnormal lymphocytes were noted. Twelve cases presented with, or developed, clinical splenomegaly. These results suggest that deletion of a gene or genes at 6q21-23 is associated with the pathogenesis of a subset of B-cell sm lym NHL that may display larger prolymphocytoid cells in the peripheral blood, but that follows a clinical course typical of other well-differentiated lymphocytic neoplasms.