Leucovorin potentiates the cytotoxicity of fluorouracil (5-FU) in experimental tumor systems and appears to enhance the effectiveness of 5-FU in patients with colon cancer. Twenty-two eligible patients (18 previously untreated) with advanced pancreatic adenocarcinoma were treated in a phase II trial of leucovorin 500 mg/m2/d for 6 days by continuous intravenous infusion with 5-FU 370 mg/m2/d by rapid intravenous injection on 5 consecutive days, beginning 24 hours after initiation of leucovorin infusion. Among the 20 assessable patients, there were no complete or partial regressions, although there was one minor response lasting 4 months. Three patients had stable disease for 5, 20, and 21 months, respectively. Median survival was 10 weeks. Toxicity was predominantly mucosal; stomatitis grade 2 or worse was seen in five patients, and diarrhea grade 2 or worse was seen in four. Hospitalization for toxicity was necessary in four previously untreated patients and three previously treated patients. The median WBC nadir was 4.6 (range, 1.4 to 9.6) x 10(3)/microL, and the median platelet nadir was 147.0 (range, 69.0 to 240.0) x 10(3)/microL. This combination of leucovorin and 5-FU did not demonstrate meaningful therapeutic activity in patients with adenocarcinoma of the pancreas and was associated with moderate to severe toxicity. It should not be considered a standard treatment for patients with this disease.