In 17 patients, large colonic lymphoid follicles were visible on air-contrast barium-enema examination (ACBE). They varied in size but were at least 4 mm in diameter and located predominantly in the rectosigmoid region. Inflammatory bowel disease was found in 13 patients (73%) and lymphoma in 1. In 100 controls without signs or symptoms of inflammatory bowel disease, the lymphoid follicles did not exceed 3 mm; they were visible radiographically in 52% of patients under 30 and 17% of those over 30. Lymphoid follicles which measure ≥ 4 mm on ACBE should suggest inflammatory bowel disease or, less likely, lymphoma or dysgammaglobulinemia.