Because of increased toxicity, full doses of gemcitabine and radiation therapy cannot routinely be given concurrently. The purpose of the present study was to determine the toxicity and response to treatment with full-dose gemcitabine given between cycles of split-course radiation therapy (nonconcurrent treatment) for inoperable periampullary adenocarcinoma. Treatment consisted of 3 6 week courses for a total of 18 weeks: 1000 mg/m gemcitabine intravenous bolus once a week x 2 weeks; 1 week break; 2 weeks of radiation therapy (1.8 Gy per fraction); 1 week break x 3. The total dose of radiation consisted of 45 Gy to the tumor + regional nodes followed by a 5.4-Gy boost. Patients were restaged at week 15 and at the completion of all treatment. Patients underwent resection if there was sufficient response. A total of 42 patients (40 pancreatic, 1 gallbladder, 1 biliary tract) were enrolled between March 1999 and July 2002. All but 2 medically inoperable patients had evidence of major vessel involvement. Median age was 63 years (range, 40-80 years). All patients were evaluable for response. There were 10 objective partial responses (24%). Six responders underwent resection with a mean survival of 18 months. Mean survival for all 42 patients was 10.3 months (range, 2.0-32.5 months; median, 9.5 months). Four patients experienced grade 3 or 4 gastrointestinal toxicity. Alternating cycles of split-course radiotherapy and gemcitabine chemotherapy permits the delivery of full doses of both modalities with acceptable tolerance. Despite the prolongation in radiation treatment time because of split-course treatment, patients with sufficient response were able to undergo resection.