Fifty-five patients with a clonal expansion of B lymphocytes in the peripheral blood were studied. According to the Kiel classification, 22 patients had chronic lymphocytic leukemia (CLL), 29 had immunocytoma (IC), two had prolymphocytic leukemia, and one had centrocytic lymphoma; one was not subclassified. Cytogenetic studies after B cell mitogen stimulation showed that six patients had an extra chromosome 12 as the sole abnormality. Another ten patients had an extra chromosome 12 together with other abnormalities. One patient had dup(12). Fifteen patients showed clonal aberrations without +12. Eleven patients showed only normal metaphases, and 12 patients were not evaluated cytogenetically. The cytogenetic subgroup pattern did not distinguish between CLL and IC patients. There was no significant difference between the CLL and IC groups as regards clinical findings and prognosis. However, the cytogenetic typing proved to be of prognostic significance. Increasing numbers of chromosomal aberrations within the cell clone were significantly associated with a poorer prognosis, ie, with impairment of survival (P = .04) and therapy-free survival (P less than 10(-4]. Patients with complex karyotypes (at least clonal aberrations) showed the poorest survival (P = .007). Patients with +12 required treatment earlier than patients with a normal karyotype (P = .01) and patients with karyotypic changes other than +12 (P = .006). These latter differences were even more pronounced when only IC patients were considered (P = .005 and P = .002, respectively). A multivariate analysis revealed that +12 was as strong an indicator of poor survival as advanced Rai or Binet stages and a stronger predictor of therapy-demanding disease.