The expression of the VAV proto-oncogene in 57 patients with chronic myeloproliferative disease (CMD), B-cell acute lymphoblastic leukaemia (B-ALL) and B-cell non-Hodgkin Lymphoma (B-NHL), and 61 with B-cell chronic lymphocytic leukaemia (B-CLL) was analysed. VAV overexpression was observed in 19.5% of cases and 81% of VAV-positive tumours also displayed VAV phosphorylation. Overexpression was not observed in B-ALL or CMD, but 13% of B-NHL and 34.4% of B-CLL patients (P = 0.002) overexpressed VAV. The overexpression and phosphorylation of VAV was detected more frequently in 13q- chronic lymphocytic leukaemias (71.4%) versus other B-CLLs (23.4%, P = 0.001). Overexpression of VAV protein is a frequent event in patients with B-CLL displaying loss of 13q sequences.