Campylobacter pylori is now known to be the most common and important cause of gastritis, and C. pylori infections have been associated with duodenal ulcer, gastric ulcer, nonulcer dyspepsia, and gastric cancer. Although it has been only possible to culture C. pylori for about 5 yr, there are already sufficient data available to allow us to develop the basic framework that relates C. pylori gastritis to the causation of peptic ulcer disease. We review the data and propose mechanisms that implicate C. pylori as an important factor in the pathogenesis of peptic ulcer disease and consider the therapeutic implications. What we now know about C. pylori begins to unravel some of the mysteries surrounding peptic ulcer disease.