We examined colonic biopsies from 39 patients with clinical and small bowel biopsy changes of celiac sprue. In 12 of 39 patients (31%), striking lymphocytic infiltration of the superficial colonic epithelium and chronic inflammation of the lamina propria were identified. These 12 cases had a mean of 30.4 lymphocytes per 100 superficial colonic epithelial cells, compared with means of 8.4 in sprue cases without colonic epithelial lymphocytosis, 4.8 in normal controls, and 32.4 in nine cases of lymphocytic colitis without concurrent celiac sprue. No case showed subepithelial collagen layer thickening. Four patients with celiac sprue and colonic lymphocytosis also had gastric biopsies; two showed gastric lymphocytosis. Intraepithelial lymphocytes at all sites were positive for the T-cell marker MT-1. These findings indicate that sprue-associated colonic lymphocytosis and lymphocytic colitis are histologically, quantitatively, and immunohistochemically indistinguishable, that the epithelial T-cell infiltration of celiac sprue occurs in glandular mucosa at all levels of the gastrointestinal tract, and that colonic subepithelial collagen deposition in patients with celiac sprue is an infrequent occurrence. These findings also suggest that gastrointestinal epithelial T-cell infiltration may be an immunologic response that is common in individuals sensitized to absorbed lumenal antigens, and that colonic lymphocytosis may occur as a response to a number of antigens, including gluten.