Drug resistance of non-Hodgkin's lymphomas may involve mechanisms of the multidrug resistance phenotype including the lung resistance protein (LRP) and the multidrug resistance protein (MRP1). To determine the clinical relevance of these multidrug resistance factors in previously untreated diffuse large B-cell lymphomas (n = 48), we studied LRP and MRP1 expression in lymphoma cells and their impact on clinical outcome. LRP and MRP1 expression were immunohistochemically assessed by means of the monoclonal antibodies LRP-56 and MRPr1, respectively. LRP was positive in 23% and MRP1 in 44% of the samples. LRP expression was associated with higher tumor stage (P = 0.03), elevated serum lactate dehydrogenase levels (P = 0.01), and the International Prognostic Index (P = 0.0001). LRP-positive patients had a lower complete response rate to polychemotherapy than LRP-negative patients (18 versus 65%; P = 0.006). Patients with LRP expression had a shorter overall survival than those without LRP expression (median of 0.9 years versus median not reached; P = 0.001). MRP1 expression was independent of clinical and laboratory parameters and had no impact on the outcome of chemotherapy or survival of the patients. These data suggest that LRP expression but not MRP1 expression is an important mechanism of drug resistance associated with worse clinical outcome in previously untreated diffuse large B-cell lymphomas. Thus, the reversal of LRP-mediated drug resistance may improve clinical outcome in diffuse large B-cell lymphoma in the future.