The Angelchik prosthesis appears to be effective in preventing gastroesophageal reflux, although its precise mechanism of action remains controversial. In a unique in vitro model, 10 freshly harvested canine esophagogastric specimens were tested for their ability to remain competent against challenges of intragastric pressure under controlled conditions of intra-abdominal pressure, longitudinal esophageal tension, lower esophageal sphincter pressure and overall length and circumference of the cardia (measure of gastric dilatation). Competency of the specimen was assessed by stepwise variation in the overall length of the sphincter, while keeping constant intraabdominal pressure (20 cm H2O), intragastric pressure (20 cm H2O), esophageal tension (physiologic), lower esophageal sphincter pressure (15 cm H2O) and degree of gastric dilatation (3 cm). With each specimen serving as its own control, the effect produced by the application of an Angelchik prosthesis was evaluated. Results consistently demonstrated that at any lower esophageal sphincter length the percent of competency was increased when the prosthesis was applied (P < 0.01). The findings indicate that the Angelchik prosthesis controls reflux by preventing unfolding of the lower esophageal sphincter when challenged by intragastric pressure.