The authors measured the serial serum carcinoembryonic antigen (CEA) levels in patients with recurrent gastric or colorectal carcinoma. Among the 45 patients with recurrent gastric carcinomas, those with lung metastases showed the most prolonged CEA doubling time (CEA-DT), followed by those with liver and peritoneal metastases, respectively. Of the 31 patients with recurrent colorectal carcinomas, those with local recurrence showed the most prolonged CEA-DT, followed by those with lung, liver, and peritoneal metastases, respectively. A positive correlation was observed for gastric and colorectal carcinomas between CEA-DT in patients with metastatic lesions and outcome (R = 0.84 and 0.93, respectively). The results of the current study suggest that postoperative measurement of CEA-DT is useful in the determination of region of recurrence of carcinoma, which would permit early surgery and chemotherapy. Measurement of CEA-DT also may be helpful for the accurate determination of prognosis.