Various genetic abnormalities are often found in B-CLL, but their relative importance in the pathogenesis and evolution of the disease has not been adequately clarified. We studied the expression of bcl-2 protein and the possible simultaneous occurrence of bcl-2 overexpression, trisomy 12 and the Rb1 and p53 gene deletions in 38 patients with B-CLL by combining immunophenotyping and dual color interphase FISH. We also looked for correlation between the genetic abnormalities and clinical parameters such as stage, disease duration from diagnosis to the time of study and overall survival. High expression of the bcl-2 protein was found in 76.3% of the patients (29/38). Trisomy 12 was found in 37% of cases (14/38) and Rb1 monoallelic gene deletion in 42% (16/38). The percentage of cells with hemizygous Rb1 deletion ranged from 13 to 18%. Monoallelic deletion of p53 was found in 29% of cases (11/38). The number of cells with only one signal ranged from 28 to 98%. Patients in stage A had on average, less than one abnormality, while patients in stage C had 2.6 abnormalities. Patients appeared to accumulate genetic abnormalities with time. Bcl-2 overexpression was found early in the course of the disease. Trisomy 12 appeared later, at about the same time as Rb1 deletion, but was not associated with adverse prognosis. Monoallelic deletion of p53 gene appeared rather late in the course of the disease and was associated with advanced stage. Despite the fact that more deaths occurred in the group of patients with three or four abnormalities and the presence of p53 gene deletion, differences in survival were not statistically significant, probably due to the limited number of patients in each group. A larger group of patients studied in a prospective manner will better clarify these issues in the future.