Peripheral blood lymphocytes from 48 consecutive, newly diagnosed CLL patients were examined by light- and electron-microscopy for the presence of nuclear cleaving or folding. On this basis, the patients could be subdivided into 2 easily separable morphological groups. In 42 patients the lymphocytes had regular round nuclei. In 6 patients (13%) the lymphocytes of peripheral blood had a highly irregular nuclear outline characterized by pronounced cleaving or folding of the nuclear membrane. In 4 of these 6 patients immunological investigations demonstrated membrane markers consistent with B-cell monoclonality. Age, sex, distribution between stages as well as mode of clinical presentation were all alike in the 2 groups of patients. In spite of this a statistically significant, highly increased mortality was observed in patients with lymphocyte nuclear irregularities. Thus, 50% of all deaths occurred in this small group of patients. These findings indicate that lymphocyte nuclear cleaving or folding represents a stage-independent prognostic parameter, which permits selection of a small but easily identified high-risk group of patients.