A study of mast cell content of the small intestinal mucosa in children with celiac disease is presented. Twenty patients with true celiac disease were studied and compared with 7 patients with transient gluten intolerance and 20 normal control patients. In healthy children we found (mean +/- SE) 142.5 +/- 16.4 mast cells/mm2. In children with active celiac disease, only 40.1 +/- 19.5 cells were found. This difference was highly significant (P less than 0.001). On a gluten-free diet for 1.5 years, the number of mast cells was 82.2 +/- 27.2/mm2 and still remained significantly depressed (P less than 0.001). Upon gluten challenge in celiac disease, the numbers fell to 58.3 +/- 32.6/mm2, while in transient gluten intolerance the numbers of mast cells attained were 102.5 +/- 22.5/mm2, near normal values. These findings indicate that during the untreated phase of celiac disease the number of mast cells is depressed. On a gluten-free diet, the number rises but does not reach normal control levels even after prolonged remission. It is suggested that even during remission of celiac disease the mast cells continue to be damaged by unidentified toxic agents.