Background: The colonic biopsy is the only reliable method for identification of microscopic colitis in patients with chronic diarrhea and normal endoscopic f indings. Methods: The Clinical Outcomes Research Initiative national endoscopic database was analyzed to determine the rate at which colonic biopsy specimens we re obtained in patients undergoing colonoscopy for the evaluation of diarrhea wi th no visible mucosal abnormality.Results: Between January 2000 and December 200 3, 5565 unique adult patients underwent colonoscopy for evaluation of diarrhea w ithout detection of any mucosal abnormality.Colonic mucosal biopsy specimens wer e obtained in 4410 (79.2%) of these patients. The rates at which biopsy specime ns were obtained differed among the sites where colonoscopy was performed; biops y specimens were obtained from more patients undergoing colonoscopy in universit y-affiliated settings(86.8%) compared with Veterans Affairs Medical Centers (V AMC) (78.5%) or community sites (78.6%) (p< 0.001). On multivariate analysis, biopsy specimens were more likely to be obtained in younger patients (OR 0.7: 95 %CI[0.6,0.8] for age >50 years vs.< 50 years), women patients (OR 1.4: 95%CI[1 .2, 1.6] in community setting; OR 4.1: 95%CI[1.6, 10.5] in VAMC setting), and p atients seen in university-affiliated medical centers (university center OR 2.1 :95%CI[1.5, 3.0] vs. community setting). Conclusions:Biopsy specimens are obtai ned in four fifths of patients with diarrhea and normal colonoscopy findings to exclude microscopic colitis. Variation in biopsy practice exists among endoscopy site types and by gender. Clear guidelines are needed for the endoscopic approa ch to these patients. Background: The colonic biopsy is the only reliable method for identification of microscopic colitis in patients with chronic diarrhea and normal endoscopic f indings. Methods: The Clinical Outcomes Research Initiative national endoscopic database was analyzed to determine the rate at which the colonic biopsy specimens we re obtained in patients undergoing colonoscopy for the evaluation of diarrhea wi th no visible mucosal abnormality. Results: Between January 2000 and December 200 3, 5565 unique adult patients underwent colonoscopy for evaluation of diarrhea w ithout detection of any mucosal abnormality. Population mucosal biopsy specimens wer e The rates at which biopsy specime ns were obtained differed among the sites where colonoscopy was performed; biops y specimens were obtained from more patients undergoing colonoscopy in universit y-affiliated settings (86.8%) compared to with Veterans Affairs Medical Centers (V AMC) (78.5%) or community sites (78 .6%) (p <0.001). On multivariate analysis, biopsy specimens were more likely to be obtained in younger patients (OR 0.7: 95% CI [0.6,0.8] for age> 50 years vs. <50 years), women OR 4.1: 95% CI [1.6, 10.5] in VAMC setting), and p atients seen in university-affiliated medical centers (university center OR 2.1 : 95% CI [1.5, 3.0] vs. community setting). Conclusions: Biopsy specimens were obtai ned in four fifths of patients with diarrhea and normal colonoscopy findings to exclude microscopic colitis. Variation in biopsy practice exists among endoscopy site types and by gender Clear guidelines are needed for the endoscopic approa ch to these patients.