Purpose: This phase II, multicenter, open-label, single-arm study evaluated the efficacy and safety of irinotecan and gemcitabine as combination chemotherapy for previously untreated patients with unresectable or metastatic pancreatic cancer. Patients and Methods: Patients received repeated 21-day cycles at starting doses of gemcitabine 1,000 mg/m2 over 30 minutes followed immediately by irinotecan 100 mg/m2 over 90 minutes, both given intravenously on days 1 and 8. Patients were evaluated for objective tumor response, changes in the serum tumor marker CA 19-9, time to tumor progression (TTP), survival, and safety. Results: Forty-five patients were treated. Eleven patients (24%) had 50% or greater reductions in tumor area. These were confirmed one cycle later in nine patients (response rate, 20%; 95% confidence interval, 8% to 32%). Among 44 patients with baseline CA 19-9 determinations, CA 19-9 decreased during therapy in 22 patients (50%) and was reduced by 50% or more in 13 patients (30%). Median TTP was 2.8 months (range, 0.3 to 10.8 months). There were significant (P < .001) correlations between proportional changes in CA 19-9 and radiographic changes in tumor area with regard to extent of change (r = .67), timing of minimum on-study values (r = .85), and tumor progression (r = .89). Median survival was 5.7 months (range, 0.4 to 19.4+ months), and the 1-year survival rate was 27%. Severe toxicities were uncommon and primarily limited to grade 4 neutropenia (2%), grade 4 vomiting (2%), and grade 3 diarrhea (7%). Conclusion: Irinotecan/gemcitabine is a new combination that offers encouraging activity in terms of radiographic and CA 19-9 response and notable 1-year survival in pancreatic cancer. The regimen was well tolerated, with minimal grade 3 and 4 toxicities and excellent maintenance of planned dose-intensity.