To identify colorectal cancers occurring after colonoscopic screening in patients at risk for Lynch syndrome.,All cancers in Lynch syndrome families on file at the Creighton University Hereditary Cancer Institute were reviewed for history of prior colonoscopy.,Of 225 individuals with 313 colon cancers, six patients from different families had colon cancers arising within 4 1/2 yr of colonoscopic surveillance. Another 17 patients had metachronous colon cancers within 5 yr of resection of their first colon cancer.,Of 225 colorectal cancer patients from Lynch syndrome families, 10.2% of patients had colorectal cancer within 5 yr of colonoscopy or colon resection. In Lynch syndrome, the potential for interval neoplasms and the malignant potential of missed diminutive adenomas may differ from cases in the general population.