Synchronous carcinoma of the colon and rectum. Reports on the incidence of synchronous carcinoma of the colon and rectum have varied from 2 to 11 per cent. The variability is a result of a lack of uniformity in criteria of diagnosis, differences in the population studied and differences in time period used. In this study, we evaluated the incidence and distribution of synchronous lesions during a recent time period before the use of colonoscopy became widespread. We reviewed the records of all patients with newly diagnosed adenocarcinoma of the colon and rectum who were operated upon at our institution between 1976 and 1981. In a total group of 1,000 patients of which 52 per cent were men, there were 54 patients or 5.4 per cent who had synchronous carcinomas. The group of patients with synchronous carcinomas were older than the group with nonsynchronous carcinomas (72.4 versus 68.8 years). There was also a higher incidence of associated benign polyps in the group with synchronous carcinomas (70 versus 30 per cent for a nonsynchronous carcinomas). The anatomic distribution of carcinomas of the colon and rectum in the group with synchronous lesions (111 in total) revealed a higher percentage of carcinomas located on the right side (29.7 versus 22.5 per cent), although the difference did not reach statistical significance. Synchronous carcinomas were located in nonadjacent segments of the colon in 37 per cent. There was no difference in stage between the groups with and without synchronous carcinomas. The preoperative identification of synchronous lesions by either colonoscopy or barium enema is important for the proper treatment of patients with carcinoma of the colon and rectum. Failure to locate these tumors may lead to the demise of the patient. 