['The diagnostic potential of residual gastric juice for development of naproxen sodium-induced mucosal damage has not been explored. We studied prostaglandin E2 (PGE2) content in residual gastric juice before and after naproxen sodium administration and assessed relationships with endoscopic mucosal damage.', 'Thirty volunteers received the recommended over-the-counter dose (660 mg/day) of naproxen sodium or placebo in this 7-day, double-blind, endoscopically controlled, cross-over study.', 'PGE2 concentration in gastric juice did not increase after placebo; naproxen significantly reduced PGE2 concentration (p < 0.001). In three subjects in whom no endoscopic changes occurred after naproxen administration, PGE2 concentration increased by 14%. In seven subjects who developed hemorrhagic changes, PGE2 concentration declined by 50% (p = 0.08). In 20 subjects who developed numerous hemorrhagic and erosive changes within the antral mucosa, PGE2 concentration declined by 70% (p < 0.001). The starting PGE2 value in subjects with severe mucosal hemorrhagic and erosive changes after naproxen was almost eightfold higher than in subjects who did not develop mucosal damage (p = 0.007) and 67% higher than in subjects with hemorrhagic changes only (p = 0.25).'] Does prostaglandin E2 content in residual gastric juice reflect endoscopic damage to the gastric mucosa after naproxen sodium administration?