A long-term postoperative carcinoembryonic antigen (CEA) follow-up study is carried out with patients having undergone primary resection of histologically proved adenocarcinomas of the gastrointestinal tract. Up to now, 122 patients who underwent curative resections, as judged from the situs and the results of histologic examinations, were followed up for tumor recurrence by computerized CEA surveillance diagrams and clinical diagnostic methods. In the cases of tumor recurrence the rise of the CEA level preceded a positive clinical diagnosis by a mean of 4 months. On the basis of the CEA time course, we selected 28 patients for second-look surgery. In all cases proof of recurrence of the disease was obtained. A local recurrence correlating with a slow CEA rise was generally resectable, metastases correlating with a rapid CEA rise were only in some cases resectable, provided that second-look surgery was carried out without delay.