Patients with chronic lymphocytic leukemia (CLL) and unmutated IgVH genes or high ZAP-70 expression have poorer prognosis than those with mutated IgVH genes or low ZAP-70 levels. This is in part related to the resistance of unmutated and ZAP-70 positive cases to treatment agents that induce apoptosis in a p53-dependent manner. It has been suggested that corticosteroids are active in CLL through p53-independent pathways. The aim of this study was to analyze the “ex vivo” response to dexamethasone in CLL cells according to the IgVH mutational status and ZAP-70 expression. Frozen lymphocytes from 60 patients with CLL were analyzed for ZAP-70 expression and IgVH mutational status (n=44). Cells were cultured and treated using fludarabine (5μgr/ml), mitoxantrone (0.5μgr/ml), FCM (fludarabine 1μgr/ml, maphosphamide 1μgr/ml and mitoxantrone 0.5 μgr/ml), and dexamethasone (5.2 μg/mL). Cell viability and apoptosis were determined by annexinV/PI staining and FACscan analysis at three different time points and conditions (0h and 24h without treatment, and 24h with treatment). The expression of glucocorticoid receptor (GR) isoforms alpha, beta and gamma was analyzed by Quantitative RT-PCR in 20 cases. Dexamethasone-induced apoptosis was significantly higher in patients with unmutated IgVH and/or high ZAP-70 expression (≥20%) than in those with mutated IgVH and/or low ZAP-70 expression (median cell viability 65% vs. 81%, respectively; p<0.001). In contrast, mitoxantrone induced higher cell toxicity in patients with IgVH mutations or low ZAP-70 expression (p=0.009). No differences in cell viability were found according to ZAP-70 expression or IgVH mutational status after “ex vivo” treatment with fludarabine or FCM (p=0.649 and p=0.055, respectively). Expression of different GR isoforms was similar in corticosteroid-responders and non-responders. In conclusion, in this study CLL cells with unmutated IgVH genes or high ZAP-70 expression showed a higher cell mortality after “ex vivo” exposure to dexamethasone than those with mutated IgVH genes or low ZAP-70 expression, with no relationship with the expression of the different GR isoforms. These data give conceptual support to trials aimed at determining the role of dexamethasone in the treatment of patients with CLL and poor prognostic features or resistant to fludarabine.