The elusive colonic malignancy. A need for definitive preoperative localization. Colonoscopy with biopsy is the standard of practice for the diagnosis of colonic malignancies. Unfortunately, the inability of endoscopy to obtain precise distance measurements from the anal verge can make localization of lesions at operation difficult. For this reason, preoperative barium enema or intraoperative colonoscopy have been advocated to further pinpoint the sites of those lesions not thought to be easily located at operation. Five patients are presented in whom malignant lesions of the colon were diagnosed and verified histologically, but were later undetectable at operation or subsequent colonoscopic examinations. Four of these patients underwent laparotomy and three received colon resections. None of these patients' tumors were identified during intraoperative colonoscopy, in the resected bowel on pathologic examination, or on follow-up colonoscopy. A fifth patient is presented who spontaneously passed a polyp containing invasive adenocarcinoma, but multiple colonoscopic examinations have failed to identify the site of the lesion. To date, none of these tumors have recurred with periods of follow-up ranging from 6 months to 2 years. These patients demonstrate a poorly documented and little understood aspect of the behavior of colonic malignancies, i.e., the ability to spontaneously regress or slough from the bowel wall. Based on these instances, localization of potentially malignant colon lesions is recommended with submucosal dye injections at initial endoscopy or with colonoscopy in the operating room immediately prior to operation. 