B-cell chronic lymphocytic leukemia (B-CLL) is the most common leukemia in adults and is characterized by the accumulation of mature B lymphocytes in the G0/G1 phase of the cell cycle, expressing B-cell related (i.e. CD19, surface immunoglobulins) and unrelated molecules (CD5 and CD23). The signal transduction pathways underlying the abnormalities of these leukemic cells are poorly understood and no data are available on deregulated cell signalling in B-CLL. Since Lyn activation plays a pivotal role in the signaling cascade triggered by BCR engagement, we investigated whether this kinase may be involved in CLL pathogenesis. In this study, we investigated freshly isolated and purified malignant B cells obtained from 40 CLL patients and we observed that the Src-kinase Lyn, the switch molecule coupling B-cell-receptor to downstream signaling, displays anomalous properties. Western blot and confocal analyses demonstrated that Lyn is overexpressed at the protein level in leukemic cells as compared to normal B-lymphocytes with a substantial aliquot of the kinase anomalously present in the cytosol of leukemic cells. While in normal B lymphocytes Lyn activation is triggered by B-cell-receptor engagement with anti-IgM antibodies, in freshly isolated leukemic cells this kinase is constitutively active and accounts for high basal protein tyrosine-phosphorylation and low responsiveness to IgM-ligation. To address the question of whether the upregulation of Lyn protein and activity plays a role in the defective apoptosis of leukemic cells, we investigated the relationship between Lyn and the cell survival of malignant lymphocytes in the presence of either dexamethazone and cyclosporin A, which are known to induce apoptosis of human lymphocytes, or PP2 and SU6656, which are selective inhibitors of Lyn. When leukemic cells were cultured in the presence of cyclosporin A or dexamethazone, a marked increase in apoptosis was observed as compared to cells cultured in medium alone, and this effect correlated with a great decrease in both basal activity and protein level of Lyn. The exposure of the leukemic cells to PP2 and SU6656 caused both the inhibition of the overexpressed Lyn activity and marked cell apoptosis. These findings suggest a direct correlation between high basal Lyn activity and defects in the induction of apoptosis in leukemic cells. They also support a critical role for Lyn in B-CLL pathogenesis and identify this tyrosine kinase as a potential therapeutic target for drugs capable of inducing apoptosis in B-CLL leukemic cells.