BACKGROUND Endoscopic surveillance of patients with ulcerative colitis aims to prevent cancer-related morbidity through the detection and treatment of dysplasia. The literature to date varies widely with regard to the importance of dysplasia as a marker for colorectal cancer at the time of colectomy. OBJECTIVE The aim of this study was to accurately characterize the extent to which the preoperative detection of dysplasia is associated with undetected cancer in patients with ulcerative colitis. DESIGN/PATIENTS/SETTING A retrospective chart review was conducted of patients undergoing surgery for colitis within the Mayo Clinic Health System between August 1993 and July 2012. MAIN OUTCOME MEASURES Patient demographics and pre- and postoperative dysplasia were tabulated. The relationship between pre- and postoperative dysplasia/cancer in surgical pathology specimens was assessed. RESULTS A total of 2130 patients underwent abdominal colectomy or proctocolectomy; 329 patients were identified (15%) as having at least 1 focus of dysplasia preoperatively. Of these 329 patients, the majority were male (69%) with a mean age of 49.7 years. Unsuspected cancer was found in 6 surgical specimens. Indeterminate dysplasia was not associated with cancer (0/50). Preoperative low-grade dysplasia was associated with a 2% (3/141) risk of undetected cancer when present in random surveillance biopsies and a 3% (2/79) risk if detected in endoscopically visible lesions. Similarly, 3% (1/33) of patients identified preoperatively with random surveillance biopsy high-grade dysplasia harbored undetected cancer. Unsuspected dysplasia was found in 62/1801 (3%) cases without preoperative dysplasia. LIMITATIONS This study is limited by its retrospective nature and by its lack of evaluation of the natural history of dysplastic lesions that progress to cancer. CONCLUSIONS The presence of dysplasia was associated with a low risk of unsuspected cancer at the time of colectomy. These findings will help inform the decision-making process for patients with ulcerative colitis who are considering intensive surveillance vs surgical intervention after a diagnosis of dysplasia.