To compare chemotherapy alone with chemoradiation therapy in stage III-IV(M0) gastric cancer treated with R0 gastrectomy and D2 lymph node dissection. The chemotherapy arm received 5 cycles of fluorouracil and leucovorin (FL), and the chemoradiation therapy arm received 1 cycle of FL, then radiation therapy of 45 Gy concurrently with 2 cycles of FL, followed by 2 cycles of FL. Intent-to-treat analysis and per-protocol analyses were performed. Between May 6, 2002 and June 29, 2006, a total of 90 patients were enrolled. Forty-four were randomly assigned to the chemotherapy arm and 46 to the chemoradiation therapy arm. Treatment was completed as planned by 93.2% of patients in the chemotherapy arm and 87.0% in the chemoradiation therapy arm. Overall intent-to-treat analysis showed that addition of radiation therapy to chemotherapy significantly improved locoregional recurrence-free survival (LRRFS) but not disease-free survival. In subgroup analysis for stage III, chemoradiation therapy significantly prolonged the 5-year LRRFS and disease-free survival rates compared with chemotherapy (93.2% vs 66.8%, P=.014; 73.5% vs 54.6%, P=.056, respectively). Addition of radiation therapy to chemotherapy could improve the LRRFS in stage III gastric cancer treated with R0 gastrectomy and D2 lymph node dissection.