Helicobacter pylori is now recognized as a frequent cause of histological chronic gastritis, and this has radically changed our understanding of this common condition. In the light of these developments, the traditional view that non-steroidal anti-inflammatory drugs are one of the common 'environmental' causes of chronic gastritis has been re-examined. Gastric mucosal biopsies have been studied from 430 patients undergoing routine upper gastrointestinal endoscopy, 99 of whom had recently been taking non-steroidal anti-inflammatory drugs. No significant association was found between the use of these drugs and either the presence of chronic gastritis or the frequency of colonization with H. pylori, although there was a strong association (P less than 0.0001) between H. pylori and gastritis. Non-steroidal anti-inflammatory drugs appear, however, to modify the inflammatory process in the gastric body, leading to a lower frequency of atrophic gastritis (P less than 0.05). The majority of peptic ulcers were associated with H. pylori irrespective of non-steroidal anti-inflammatory drug use, but there was a higher frequency of H. pylori negative ulceration in the patients who had used these agents (P less than 0.04). Peptic ulceration was uncommon in the absence of either H. pylori or recent non-steroidal anti-inflammatory drug use.