The incidence of synchronous cancer has been reported to be 1.5% to 7.6%. A retrospective review of all colorectal cancer patients was conducted to determine the incidence of synchronous cancers. 47 synchronous cancers were identified in an operative series of 2586 patients with an incidence of 1.8%. The patients were divided into two groups based on preoperative diagnostic examinations. The first group included 1608 patients examined by double contrast x-ray barium enema (1141) and/or single contrast barium enema (381) and/or rigid sigmoidoscopy (112). The second group included 978 patients examined by double contrast barium enema (459) and/or flexible sigmoidoscopy (631) and/or colonoscopy (389). The incidence of synchronous tumors was 1.6% for patients evaluated only with double contrast roentgenographic study and 4.1% for patients who underwent preoperative colonoscopy. Fifty percent of synchronous cancers detected by colonoscopy were missed at double contrast x-ray barium enema. While twenty two percent of synchronous tumors detected by barium enema were located in different surgical segments and were all "advanced" tumors, when preoperative colonoscopy was performed, 56.2% of synchronous tumors were not located in the same surgical segment, and 66.7% of these were "early" cancers. None of the patients who underwent preoperatively colonoscopy developed a metachronous tumor within 3 years. One percent of patients undergoing flexible sigmoidoscopy or double contrast barium enema developed a second tumor 3 years after surgery. An extended use of preoperative colonoscopy in the diagnosis of colorectal cancers, to increase the detection od synchronous tumors and to decrease the incidence of the so-called "early" metachronous cancers, is stressed.