Fluorouracil (Adrucil) has been used for more than 20 years to treat metastatic colorectal carcinoma and has provided significant palliation of symptoms to some patients. Investigators are attempting to exploit pharmacologic aspects of fluorouracil that will provide further benefits. Prolonged infusion of the drug appears to overcome the problems of its brief half-life and the small percentage of cancer cells that are susceptible to the drug at any one time. The addition of leucovorin increases fluorouracil's ability to inhibit thymidylate synthase and thus colon cancer cell replication. It is hoped that the increased response rates produced by these advances will produce increased survival rates as well.