The effect of 1 week of treatment with indomethacin 150 mg/day on human gastric prostanoid synthesis was correlated with its effect on gastric and duodenal mucosa. Before and following 1 week of treatment, endoscopic appearance of the mucosa was evaluated and scored. Following 1 week of treatment with indomethacin, antral PGE2 and 6-keto-PGF1 alpha were significantly lower than in normal subjects, but similar in patients with significant or with no mucosal damage. Co-treatment with ranitidine 150 mg b.d. or with cimetidine 400 mg b.d. reduced the mean mucosal damage score but did not affect gastric prostanoid synthesis, which was similar irrespective of the presence or absence of mucosal damage. It is therefore suggested that there is no correlation between indomethacin-induced inhibition of gastric prostanoid synthesis and its induction of mucosal damage.