Some studies have revealed gender bias against women in various aspects of medical care. There is no substantial evidence of gender bias in patients undergoing cancer evaluations, specifically colorectal cancer screening and diagnosis of colorectal complaints. This study was designed to examine the role of gender bias related to patients undergoing flexible sigmoidoscopy. At the University of South Florida, we conducted a retrospective study of 1910 patients at three distinct flexible sigmoidoscopy clinics over several years, through 1992. The proportions of male and female patients who underwent the procedure for indications of either screening for colorectal cancer or the diagnosis of colorectal complaints were determined. These proportions were compared with the respective male and female patient proportion from the total number of currently active patients at each site who were eligible to have the procedure for an appropriate indication. At all three sites, a significantly smaller proportion of women (p < 0.01) underwent the procedure than expected. This was true for both screening and diagnostic indications. Conversely, at all sites significantly more men (p < 0.01) underwent the procedure for both indications. The results of this study suggest gender bias against women for patients undergoing flexible sigmoidoscopy for both screening and diagnosis. This bias may adversely affect the lethality of colorectal cancer in women. It is important to determine if such biases are influenced by the physician's recommendation or mainly due to patient attitudes.