Digesting the facts on stomach ulcers A stomach ulcer is a small erosion in the stomach lining or the first 12 inches of the gastrointestinal tract. The explanation of how you contract an ulcer has evolved from what seemed like an educated guess 50 years ago to something based more in scientific fact today. For more Science Nation, go to http://www.nsf.gov/news/special_reports/science_nation/index.jsp