Multidrug resistance (MDR) is a challenge in cancer treatment. One of the most studied mechanisms is P-glycoprotein (P-gp), which acts as a drug efflux pump, with decreased intracellular accumulation of drugs. It still needs to be clarified whether P-gp expression has a significant impact on non-Hodgkin's lymphoma treatment response, but a poor outcome has been reported in patients with positive P-gp expression. AIDS-related lymphomas have aggressive behavior, and although a complete response could be achieved, relapse is not uncommon. In an attempt to determine a possible relationship between MDR and poor outcome in this population, histologic samples obtained from 45 non-Hodgkin's lymphoma HIV-infected patients without previous cytotoxic therapy were submitted to immunohistochemical analysis using monoclonal antibody C494 specific for the MDR-1 isoform of P-gp. Samples from 27 patients (60%) were positive. Response to treatment (P=0.02) and overall survival (P=0.001) were significantly lower in patients with positive P-gp expression. In patients having achieved complete remission, the median disease-free survival (DFS) was not reached; the mean DFS was 57.2 months with DFS rates of 72.9% in three years. Our results show that P-gp is expressed before treatment of non-Hodgkin's lymphoma of HIV patients, and is related to poor response to treatment and overall survival.