Saglam A, Hayran M, Uner AH, Immunohistochemical expression of multidrug resistance proteins in mature T/NK-cell lymphomas. APMIS 2008;116:791–800. Multidrug resistance (MDR) is defined as resistance of tumor cells to a wide spectrum of structurally and functionally unrelated drugs. One of the most important mechanisms in mediating MDR is that involving cellular drug efflux transporters. Drug resistance is a common and formidable obstacle to therapy in mature T/NK-cell lymphomas and the MDR phenotype is thought to be one of the contributing mechanisms. In this study we assessed the immunohistochemical expression of P-gp (P-glycoprotein), MRP-1 (multidrug resistance associated protein 1), BCRP (breast cancer resistance protein) and LRP (lung resistance protein) in 45 mature T/NK-cell lymphomas diagnosed at our hospital. We detected P-gp expression in 31% (13/42), MRP-1 expression in 74% (31/42), BCRP in 78% (32/ 41) and LRP in 59% (26/44) of the cases. These findings show that our T/NK-cell lymphoma cases display high frequency of MDR protein expression.