Abstract The radiographic method of diagnosing intestinal lactase deficiency during consecutive routine upper gastrointestinal examinations was assessed. Of 260 patients given a barium-lactose mixture, 15 (6%) demonstrated radiographic evidence of lactase deficiency; 9 were confirmed by enzyme analysis, 2 had abnormal oral lactose tolerance tests, 2 had normal enzyme levels, and 2 refused further study. The 2 false-positive results occurred in patients with gastrectomies. One patient was aware that symptoms were produced by drinking milk, and 3 patients had symptoms compatible with a lactase deficiency. The diagnosis had not been considered by the patients' physicians.