OBJECTIVE Although the finding of trisomy 12 in B-cell malignancies has been extensively documented especially in B-CLL, little is known about the clonal involvement of different tissues and there are few sequential studies documenting the development of trisomy 12 during the course of the disease. The aim of this study was, therefore, to: 1) ascertain the prevalence of trisomy 12 by FISH; 2) correlate the findings of trisomy 12 with hematologic and clinical features; 3) study the trisomy 12 positive clone during the course of the disease, and 4) compare findings of trisomy 12 in different tissues. METHODS This a study of an unselected population of 118 patients with CLL or other B-cell disorders in leukemic phase from a defined geographic area. Trisomy 12 was detected by FISH. RESULTS Trisomy 12 was found in 18 patients (15%). The aberration was significantly more common in morphologically atypical CLL (aCLL) (24%) and CLL/PL (67%) compared to typical CLL (2%) (p < 0.001). aCLL cases had predominantly lymphocytes with lymphoplasmacytoid features. Sequential studies of peripheral blood showed an increase in the proportion of trisomic cells during the observation time, mostly associated with disease progression. None of the initially trisomy 12 negative patients acquired the aberration during follow-up. The percentage of lymphocytes exhibiting trisomy 12 was significantly (p < 0.05) higher in the bone marrow than in peripheral blood. CONCLUSIONS Trisomy 12 might define a distinct disease entity with atypical lymphocytes in chronic leukemic B-cell disorders.