CD26 (dipeptidyl peptidase IV, DPP IV) is widely expressed by T and natural killer (NK) cells, epithelial and endothelial cells of different tissues, and it is strongly upregulated in activated B-cells; moreover it plays a regulatory role in the neoplastic transformation and progression of various types of tumours. CD26 expression was evaluated by means of flow cytometry in various peripheral B-cell lymphoid tumours: 12 follicular and 12 mantle cell lymphomas, 20 multiple myelomas (MMs), 12 hairy cell leukaemias (HCLs), 112 chronic lymphocytic leukaemias (CLLs), 20 CD5(negative) B-cell chronic lymphoproliferative diseases (CD5(neg) B-CLPDs) and 12 diffuse large cell lymphomas (DLCLs). CD26 expression was absent or barely detectable in follicular and mantle cell lymphomas, high in MMs and HCLs, and variable in CLLs, in CD5(neg) B-CLPDs and in DLCLs. CD26 significantly correlated with CD49d and CD38 expressions (p < 0.0001) in B-CLLs, and there was a significant correlation between CD26 and ZAP-70 expressions or IgVH mutational status (p < 0.0001). After a median follow-up of 36 months, 65 B-CLL patients were treated; taking 10% as the best CD26 cut-off value, Kaplan-Meier curves revealed a significantly shorter time to treatment in the CD26-positive cases (p < 0.0001). Overall, our data indicate that CD26 expression may identify subsets of B-CLL patients with an unfavourable clinical outcome in terms of therapeutic need, thus suggesting its potential role as a marker (together with CD38 and CD49d) in a future routine cytofluorimetric panel to be validated for the prognostic stratification of B-CLLs.