In this phase II trial, we used the combination of gemcitabine and 5-fluorouracil (5-FU) to treat 26 patients: 17 (65%) with advanced pancreatic adenocarcinoma and 9 (35%) with advanced biliary tract adenocarcinoma (10 locally advanced and 16 metastatic); 15 (57.7%) male and 11 (42.3%) female; median age 58 (range, 39-68); median performance status 2 (range, 1-3). A total of 102 cycles were administered (median, 4 per patient). There were 8 objective responses, plus 1 complete response not confirmed by second-look laparotomy, thus the overall objective response rate was 30.7% (95% CI 12%-47%). Among the patients with biliary tract carcinoma, 33% (3/9) had PR. Six (23%) patients had stable disease (SD). All 8 responders and 3 of the patients with SD experienced clinical benefit (42%). The median overall survival was 9 months (range, 6-38), and the 1-year survival rate was 30%. The regimen was very well tolerated. One patient developed reversible World Health Organization grade IV febrile neutropenia. We observed grade III neutropenia in 11 (11%) cycles; grade III thrombocytopenia in 7 (7%) cycles; grade III mucositis in 7 (7%) cycles; and grade III diarrhea in 10 (10%) cycles. Asthenia grades I and II occurred in 30% of cycles and flulike syndrome grade II in 11 (11%) cycles. The combination of gemcitabine and 5-FU in patients with advanced pancreatic or biliary tract cancer produces promising activity and tolerability with the added potential for clinical benefit, and thus warrants further investigation.