OBJECTIVE The aim of this study was to determine whether anatomic factors such as body mass index (BMI) impacts the success rate of cecal intubation during colonoscopy. METHODS We retrospectively reviewed the cecal intubation rate of 2000 colonoscopies performed at our institution from March 1997 to March 1999. The analysis sample was composed of charts for all incomplete procedures and a sample (23%) of complete examinations that were randomly selected. Data collected included age, gender, height, weight, bowel habits, abdominal surgery, psychiatric medication use, the presence of diverticular disease, amount of sedation administered, and location and reason for halting the examination. Patients were divided by BMI: thin (BMI < or = 22.1), average weight (BMI > 22.1-25.0), overweight (BMI = 25.1-29.9), and obese (BMI > 30). RESULTS Colonoscopies in women had a lower adjusted completion rate (94.8%) than in men (98.2%) (p < 0.005). A low BMI in women was predictive of an incomplete examination (p < 0.001). Factors that did not predict incomplete examinations in women included age and previous hysterectomy. The small number of male patients with an incomplete examination (n = 16) precluded accurate identification of any factors. CONCLUSIONS Women with a low BMI (especially < 22) were more likely to have an incomplete procedure. This finding may have implications for colorectal cancer screening in female patients.