RATIONALE: Drugs used in chemotherapy, such as tegafur-uracil, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Giving chemotherapy after surgery may kill any tumor cells that remain after surgery. Sometimes, after surgery, the tumor may not need more treatment until it progresses. In this case, observation may be sufficient. It is not yet known whether chemotherapy is more effective than observation in treating colorectal cancer.