The relationship of coarse gastric and duodenal folds to duodenal ulceration and to acid secretory capacity was examined in a series of 44 patients. Subjective assessment of coarse gastric folds correlated well with acid secretion. However, the factors responsible for subjective assessment of coarse folds were not capable of simple numerical expression. In contrast, coarseness of duodenal (and jejunal) folds could be assessed by simple measurements. The folds in the duodenum and jejunum were significantly wider and deeper in ulcer patients than in those with radiologically negative dyspepsia. The ulcer patients also had, on average, a higher maximal acid secretory capacity than the others. But the correlation between maximal acid secretory capacity and duodenal fold dimensions was poor for individual cases. This suggests that factors other than maximal acid secretory capacity determine the radiological appearance of coarse duodenal folds.