B-cell chronic lymphocytic leukemia (CLL) follows a heterogeneous clinical course, for which several biological markers may predict clinical outcome. Cytogenetic aberrations are considered major prognostic indicators for predicting the survival of CLL patients. Given the difficulties in obtaining abnormal metaphases in CLL, fluorescent in situ hybridization (FISH) with specific probes is generally used to detect the most frequent abnormalities. To determine the best strategy for identifying cytogenetic abnormalities, we compared results obtained by FISH analysis on peripheral blood mononuclear cells with those obtained by FISH after culture with mitogens. We studied 46 CLL patients selected from two different institutions. The most frequent structural aberrations leading to loss of genetic material were loss of the 13q14 region, in 32 cases (70%), and loss of TP53 in 11 cases (24%). Of the 46 cases patients, 10 patients (21.7%) had deletion of the ATM locus at 11q22 and 8 patients (17%) had trisomy 12. Results could be interpreted as discordant in four cases in which the abnormal clone was small and both values were around the threshold. FISH performed on stimulated cells detected the same frequency of abnormalities as FISH performed without culture. This equivalence between the two techniques allows performing both conventional cytogenetic and FISH analyses on the same sample.