The purpose of this study was to assess the efficacy and toxicity of intensity-modulated radiation therapy (IMRT) in the treatment of gastric cancer. Seven patients with gastric cancer were treated with IMRT. Six patients (all Stage III) received post-operative chemoradiotherapy with concurrent 5-fluorouracil and leucovorin. One received planned pre-operative radiation, though did not proceed to surgery. All patients were planned to receive 50.4 Gy in 1.8 Gy fractions. IMRT planning was compared with opposed anterior-posterior: posterior-anterior (AP/PA) and 3-field conventional three-dimensional plans. When compared with either AP/PA or 3-field plans, IMRT significantly reduced the volume exceeding the threshold dose of the liver and at least one kidney. Target coverage with IMRT was excellent, with 98+/-1% of the target receiving >or=100% of the dose. Compared with AP/PA and 3-field plans, IMRT plans had a greater percentage of target receiving the prescribed dose, but also a greater volume receiving >110% of the dose. IMRT was well tolerated; no patients developed acute gastrointestinal toxicity greater than grade 2. All seven experienced grade 2 nausea, three had grade 2 diarrhoea and two had grade 2 oesophagitis. Weight loss ranged from 0-12% (mean 6.1% and median 5.8%). IMRT in the treatment of gastric malignancies reduces the mean and above threshold doses to critical normal tissues. In an initial cohort of seven patients, 50.4 Gy delivered by IMRT is well tolerated and safe.