This study investigated the spectrum of gastric mucosal pathology, including the prevalence of reactive gastritis in patients on non-steroidal anti-inflammatory drugs (NSAIDs). The histological findings were correlated with upper gastrointestinal symptom status and endoscopic findings and were also compared with the histological appearances of the gastric mucosa in a corresponding age-matched control group of 75 patients not receiving NSAIDs or any other drug therapy. Reactive gastritis of the gastric antrum was more common in the NSAID group and was observed in 34 patients (45.3%), as an isolated phenomenon in 24 patients (32%) and with evidence of coexistent chronic gastritis in 10 patients (13.3%). In the control group reactive gastritis of the antrum was seen in 10 patients (13.3%), as an isolated finding in eight cases (10.7%) and with accompanying chronic gastritis in two cases. Chronic antral gastritis of usual type was observed in 36 patients on NSAIDs (48%) and Helicobacter-like organisms were identified histologically in 18 of these (50% carriage rate). These organisms were not seen in any of the patients in whom the picture of reactive gastritis was present. In the control group chronic antral gastritis was seen in 51 patients (68%) with organisms in 34 (66.6% carriage rate). No correlation was found between the presence or absence of upper gastrointestinal symptoms, endoscopic findings and the histological appearances of the gastric mucosa. We conclude that NSAIDs are an independent cause of reactive gastritis in the antrum and do not appear to alter gastric mucosal colonization by Helicobacter-like organisms.(ABSTRACT TRUNCATED AT 250 WORDS)