The pathogenesis of cap polyposis remains unknown. Here, we report a patient with cap polyposis that developed simultaneously in the colon and stomach, and which regressed after Helicobacter pylori eradication. A 63-year-old man was diagnosed as having cap polyposis with mucoid stool, diarrhea, and bleeding on defecation. Following 5 weeks of total parenteral nutrition, his symptoms and hypoproteinemia improved and he was discharged, although follow-up colonoscopic findings revealed no improvement. Subsequent gastroscopy revealed diffusely erosive polyps with cap-like "fur" from the angle to the antrum of the stomach, similar to the lesions observed in the colon. Because H. pylori infection was demonstrated in the stomach, eradication therapy was administered. After this treatment, his symptoms immediately disappeared, and the polypoid lesions in the colon and stomach had completely disappeared 8 months later. Because there have been no previous reports of a relationship between H. pylori and cap polyposis, this case is of great interest.