Aims: The purpose of follow-up of patients after curative surgery for colorectal cancer is to improve survival and quality of life. However, there has been no standard system of follow-up. The aim of this study is to investigate an optimal follow-up system of stage I colorectal patients. Patients and Methods: Consecutive 1599 stage I patients, who underwent curative resection at 16 institutions in Japan of the Study Group on Postoperative Follow-Up of Colorectal Cancer Patients from January 1991 to December 1996, were enrolled in this study. Patients with T1 cancer, who underwent only endoscopic resection, were excluded. A common follow-up system consisted of carcinoembryonic antigen measurement every 3 months, ultrasonic imaging for liver every 3-6 months, chest X-ray every 6 months and colonoscopy every 1-2 years. Where recurrences were suspected, CT or/ and MRI were applied. The median follow-up period was 89 months. The 5-year overall survivals of T1N0 and T2N0 cancer were 94.3% and 90.5%, respectively. Recurrence rates and sites, treatments for relapse and outcome of re-resection were investigated. Results: The recurrences were observed in 20/ 814 (2.5%) patients with colon cancer and 57/ 785 (7.3%) with rectal cancer. The median periods of the recurrences were 26 months. Cumulative appearance rates of recurrence were 22%, 68%, and 92% within 1, 3, and 5 years after surgery, respectively. 4 patients of 488 (0.8%) with T1N0 colon cancer had recurrences. The median periods were 18 months. The recurrence sites were liver in 1 and multiple hematogenous metastases in 3. All patients were passed away by a specified cancer. 16 of 326 (4.9%) with T2N0 colon cancer had recurrences. The median periods were 23 months. The recurrence sites were liver in 10, lung in 1, multiple hematogenous metastases in 3, and unknown in 2. Only 3 patients with liver metastasis survived more than 3 years after re-resection. 14 of 338 (4.1%) in T1N0 rectal cancer had recurrences. The median periods were 39 months. The sites were liver in 2, local recurrence in 6, lung in 4, and unknown in 4. 43 of 447 (9.6%) with T2N0 rectal cancer had recurrences. The median periods were 23 months. The sites were liver in 13, lung in 5, local in 15, multiple hematogenous metastases in 6, and unknown in 4. Conclusions: There were small numbers of recurrences in T1N0 colon cancer, and if once recurrence, these prognoses were very poor. Follow-up may not benefit for T1N0 colon cancer. In T2N0 colon cancer, it may be reasonable that the follow-up system is target for liver recurrence. T1N0 and T2N0 rectal cancer may be recommended to be followed up for liver, lung, and local recurrences.