Gastric duplication occurred in a 3-year-old white male infant with episodic vomiting, abdominal pain, and radiologic findings consistent with hypertrophic pyloric stenosis. A cystic mass in the wall of the gastric antrum, compressing the pyloric canal, was excised without complication. The cyst was lined by gastric mucosa supported by a submucosa and layers of smooth muscle. The pertinent literature on gastric duplication and the theory of its formation, complications, and methods of surgical treatment are summarized. To our knowledge, this is the third case of gastric duplication to be reported in the English literature that appeared clinically and radiologically as hypertrophic pyloric stenosis.