The progression, age-behaviour and profiles of chronic gastritis were studied in 460 patients with active gastric or duodenal ulcer, and in 226 patients with ulcer scar. The results were compared with those obtained from a sample of subjects representing the general population. In patients with ulcer or ulcer scar, the progression of chronic gastritis was more rapid in antrum than in body mucosa, and was more rapid in patients with proximal ulcer in those with distal ulcer or in controls. In the body the progression of gastritis was significantly slower in patients with duodenal or juxtapyloric ulcer than in patients with proximal ulcer or in nonulcer controls: body gastritis tended to remain on the same level at all ages whereas it showed a steady progression with age in the nonulcer controls. The degree of gastritis showed a tendency to increase along the shift of ulcer to more proximal in the stomach; the prevalence of gastritis of pure B type (moderate or severe atrophy in antrum, but no atrophy in body mucosa) correspondingly increased along this shift. Severe antral atrophic gastritis was found in 7 per cent of proximal active gastric ulcers and in 20 per cent of proximal ulcer scars. The progression of antral and body gastritis was on the whole more rapid in males than in females irrespectively of the location of ulcer. We conclude that gastritis in different types of ulcers shows characteristic patterns of distribution, dynamics and progression with age. The high prevalence of severe grades of atrophic antral gastritis may also be of significance in regard to the pathogenesis of gastric cancer.