Prostaglandin E was measured by radioimmunoassay in endoscopic biopsy specimens obtained from 7 normal healthy volunteers, 28 patients with a benign gastric ulcer, and 10 with a malignant gastric ulcer. Biopsy specimens were taken from the antral and body mucosa and from the ulcer edge in patients. The median value in normal antrum specimens was 5.15 (4.44-8.40) ng PGE/mg protein which was more than the body with 3.50 (1.86-9.28), p less than 0.05. In the 28 patients with benign gastric ulcer the ulcer edge contained 2.33 (0.48-7.67) ng PGE/mg protein which was more than the antrum with 1.06 (0.30-4.17) or the body with 0.97 (0.20-7.67), p less than 0.01 respectively. The antral and body levels in patients with gastric ulcers were lower than those found in the normal volunteers, p less than 0.01. There was no difference between the levels found in patients with malignant as opposed to benign gastric ulcer. In patients with gastric ulcer lower levels of PGE were found in body mucosa with atrophic gastritis 0.83 (0.53-2.66) ng PGE/mg protein as compared with histologically normal body mucosa 2.55 (0.63-7.67), p less than 0.05. These results suggest that mucosal PGE levels are reduced in patients with gastric ulcer disease and that this may be due to the presence of atrophic gastritis.