Since the thymus is thought to play an important role in the pathogenesis of ulcerative colitis, thymic abnormalities were studied on 18 cases of ulcerative colitis by using pneumomediastinography and histopathological examination. We have compared the thymic abnormality of ulcerative colitis with that of 104 cases of variety of autoimmune diseases. The denser and larger thymic shadow was seen in ulcerative colitis and this finding was same as in other autoimmune diseases. But the thymic size in ulcerative colitis was the largest in all examined autoimmune diseases. The incidence of thymic lymphoid follicle formations in ulcerative colitis was significantly higher than that of accidental death and this high incidence of lymphoid follicle formation is similar to other autoimmune diseases. We suggested that ulcerative colitis might be caused by the abnormal thymus dependent system.