In situ hybridization was performed to study the clinical significance of trisomy 12 in fifty patients with B-cell chronic lymphocytic leukemia at various stages of disease. Trisomy 12 was detected in 12%-65% (median 53%) of the circulating neoplastic cells in seven out of 20 patients with advanced Binet stage C disease. In contrast, 22 patients with Binet stage A and eight patients with Binet stage B disease were found to be negative for trisomy 12. As occurrence of trisomy 12 was associated with the presence of B-symptoms and hepatosplenomegaly, its association with advanced disease was further considered. In addition, atypical morphology was a common finding in trisomic patients who also displayed higher serum levels of soluble CD25 than patients without trisomy at Binet stage C. No significant differences were detected in serum levels of soluble CD8 and of soluble CD23. No correlation with a lymphocyte doubling time of < 12 months, marked lymphadenopathy, or prior treatment was apparent. However, refractoriness to treatment was evident more frequently in trisomic than in non-trisomic patients (p < .05). In conclusion, trisomy 12 in B-cell chronic lymphocytic leukemia appears to occur predominantly in advanced and symptomatic disease with atypical morphology. It could indicate a high risk for treatment failure thus serving as a marker of poor prognosis in this disease.