In a review of cases of colorectal cancer presenting to St. Mark's Hospital over the 16-year period 1970-85, 59 patients were found to have a synchronous carcinoma (3.4 per cent). Although 82 per cent of these synchronous tumours were distal to the splenic flexure (and hence within reach of a 60 cm flexible sigmoidoscope) only 42 per cent were detected pre-operatively: the remaining tumours were noted at surgery (24 per cent) or found incidentally on pathological examination of the resected specimen (34 per cent). Histological examination of these synchronous lesions revealed a high proportion with favourable stage (Dukes' A-75 per cent) and grade (well or moderate differentiation-90 per cent). Over the same 16-year period, 10 patients presented with an 'early' metachronous lesion (less than 3 years from initial surgery). Review of these cases noted negative findings on the initial barium studies in four patients and a failure to conduct full examination of the colon at initial presentation in the remaining six. It is concluded that full examination of the colon in all patients presenting with primary colorectal cancer is mandatory and that, in the light of this experience and recent reports in the literature, this should be by pre- or peroperative colonoscopy.