Among 873 patients undergoing upper gastrointestinal endoscopy for various reasons over a two-year period, four had a loss of Kerckring's folds in the descending duodenum. Endoscopic duodenal biopsy in all four patients revealed subtotal villous atrophy due to celiac disease. We undertook a prospective study to evaluate the extent to which this finding predicted celiac disease in 65 consecutive patients referred for intestinal biopsy. Duodenal folds were absent or markedly decreased in 15 of 17 patients with subtotal villous atrophy and in 8 of 48 patients with partial villous atrophy or normal duodenal mucosa, giving a sensitivity of 88 percent and a specificity of 83 percent for this endoscopic finding with respect to celiac disease. We recommend that all patients undergoing upper gastrointestinal endoscopy be examined for the loss or reduction of duodenal folds and, should this be found, that the examination include duodenal biopsy. The value of this procedure as an aid in the diagnosis of celiac disease should be particularly great in patients with minimal, transient, or unrelated symptoms.