Four patients with an established diagnosis of Cowden's disease underwent barium meal study, upper gastrointestinal endoscopy, barium enema examination, and colonoscopy. In three, the esophagus was affected by small protrusions, which were diagnosed as glycogenic acanthosis. Numerous hyperplastic polyps were found in the stomach in three patients, and in one an inflammatory fibroid polyp was also detected. Either lymphangiectasia or lymphoid polyps were found in the duodenum in two patients. In all patients, the colon showed polyps that varied in histology and included adenoma, hamartomatous polyp, and ganglioneurofibroma. In addition, jejunal lymphangiomas were found in one of the three patients in whom the small intestine could be precisely evaluated. These findings suggest that the gastrointestinal involvement in Cowden's disease is characterized by various benign lesions, especially esophageal glycogenic acanthosis, numerous gastric hyperplastic polyps, and multiple hamartomatous polyps in the rectosigmoid colon. Detection of these gastrointestinal manifestations may lead to early diagnosis of this potentially malignant disease.