BACKGROUND Failures of diagnosis of colorectal cancer by colonoscopy, barium enema, and flexible sigmoidoscopy have been demonstrated using various techniques. A relative assessment of these diagnostic tests for patients with colorectal cancer has not been reported. This study was designed to determine relative rates of failures for these tests when applied to diagnosis of colorectal cancer. METHODS We created a database of patients with colorectal cancer diagnosed between 2000 and 2005. Records were reviewed for the results of colonoscopy, barium enema, and flexible sigmoidoscopy in the 3 years prior to diagnosis. An examination that was negative for cancer with no immediate follow-up was defined as a failure of diagnosis, either from inaccurate observation, failure to examine the entire colon, or failure of timely follow-up. The failure rates were compared. RESULTS Three hundred seventy-nine patients, who had 421 examinations, were analyzed. The diagnosis of colorectal cancer failed in 60 of 379 patients (16%). These 60 patients had 71 examinations that failed to make the diagnosis: 25 of 282 colonoscopies (9%), 16 of 79 barium enemas (20%), and 30 of 60 flexible sigmoidoscopies (50%). These differences were statistically significant. CONCLUSIONS Failure rates for colonoscopy, barium enema, and flexible sigmoidoscopy were 9%, 20%, and 50%.