Adenocarcinomas of the pancreas and biliary tract are highly malignant neoplasms, which are found in the advanced stage. Chemotherapy commonly plays a palliative role in the treatment of pancreatic and biliary tract cancers. 5-Fluorouracil (5-FU) is the most widely studied single agent; the response rate of 5-FU is only 20%. Recently, some reports presented interesting results, in which 5-FU, modulated with levofolinic acid (leucovorin), was active in patients with colorectal cancer. In relation, we performed a phase II study of 5-FU, modulated with leucovorin, in patients affected by advanced pancreatic or biliary tract cancer. Fifty-one patients with nonresectable carcinomas of the pancreas or biliary tract admitted to Korea University Hospital between May 1995 and December 1998 were included in this study. Chemotherapy consisted of leucovorin 25 mg/m2/day by 2-hour intravenous infusion, followed by 5-FU 375 mg/m2/day by bolus intravenous infusion, from day 1 to day 5. The treatment was repeated every 3 to 4 weeks. A total of 51 eligible patients with advanced adenocarcinoma of the pancreas or biliary tract were enrolled. Of 23 enrolled patients with pancreatic adenocarcinoma, one patient showed complete remission with a survival duration of 13 months (response duration was 9 months). Three patients had partial responses (PRs) with survival times of 6, 12, and 15 months, respectively. The overall response rate was 17.4% (95% confidence interval [CI], 7.2%–36.2%). The median time of overall survival was 6 months (range: 1–15 months). Of 28 enrolled patients with biliary tract cancer, complete responses were observed in 2 patients (7.1%) with survival time of 14 and 16 months, respectively. Seven patients had PRs with a median survival of 8 months. The overall response rate was 32.1% (95% CI, 20.3%–57.5%). The median time of overall survival was 6 months (range: 1–16 months). The most prominent toxicity was mucositis. Hematologic toxicity was less severe. 5-Fluorouracil in modulation with intravenous leucovorin is well tolerated by patients with stage IV pancreatic adenocarcinoma or biliary tract cancer. Although the response rate for patients with pancreatic adenocarcinoma is not better than that achieved using 5-FU monochemotherapy, the 32.1% overall response rate achieved in patients with biliary tract cancer suggests that 5-FU modulation with leucovorin is active in biliary tract cancer.