Cancer of the colon and rectum are the third most common cancers in Canadian males and females. The initial therapy of colorectal cancer is surgery to remove the cancer and nearby lymph glands. If the cancer has spread to the lymph glands there is a high chance that the cancer will come back. To reduce the risk of the cancer recurring, patients are treated with an anticancer drug 5-fluorouracil (5FU) in combination with a vitamin leucovorin (LV). This study will determine if a simple blood test can predict which patients are at risk for developing side effects from this chemotherapy. In addition, participants of this study will be followed to determine if this same blood test will predict which patients will have their cancer relapse.