Peripheral blood samples from 61 patients (36 male, 25 female) with all stages of B-type chronic lymphocytic leukemia (CLL) were studied for MDR1 phenotype using monoclonal antibodies and rhodamine-123 dye exclusion, a functional assay of MDR1 expression. The duration of the disease varied from 1 month to 22 years at the time of initial study. Overall, 74% of the patients were positive for rhodamine-123 exclusion. When analyzed by gender, significantly more men than women were positive (89% versus 48%, p<0.001). There were more positive men than women for every stage of the disease. Female patients were found to be either MDR1 phenotype positive or negative at any stage of the disease. In contrast, all male patients with early (stages 0–II) disease were MDR1 phenotype positive. One early-stage (stage II) male patient converted from rhodamine-efflux positive to rhodamine-efflux negative as he progressed from stage-II to stage-IV disease. We suggest that some of the differences in disease biology of male versus female CLL patients (women having a more benign course) may be due to gender-dependent differences in drug-resistance gene activity, including MDR1. Our results also emphasize the need to take into account gender in evaluating the clinical course of patients with CLL.