Multidrug resistance (MDR) is the major obstacle to cancer chemotherapy. MDR phenotype is mainly related to the over-expression of MDR1 gene, being responsible for tumor resistance to several chemotherapeutic drugs. It has been suggested that MDR1 expression is redox-regulated and we have recently described a pro-oxidative effect of retinol. Here we tested the therapeutic use of retinol as a modulator of MDR1 gene expression in tumor cell lines, and verified in situ the enhancement of anticancer drug efficacy. Two human colorectal adenocarcinoma cell lines (HT29, SW620) with different degrees of MDR1 expression were used. Cells were pre-treated with a sublethal dose of retinol and then challenged with the etoposide (VP16) drug. The drug GI50 was assessed by SRB method and levels of MDR1 expression were determined by semi-quantitative rtPCR. Retinol treatment caused a 40% decrease in MDR1 expression and increased VP16 toxicity. MDR1 expression and drug sensitivity were restored to control values when mannitol (a hydroxyl radical scavenger) was co-administrated. Our data point a role to the use of retinol as an adjuvant in the treatment of tumors with MDR phenotype.