Lymph t~ollicles in the Thymus in Cases o/Ulcerative ColitisSummary. Histological examination of the thymus from eight patients with severe, therapy-resistant ulcerative colitis subjected to the therapeutic thymectomy revealed in three cases lymph follicles with florid germinal centers in the medulla of the thymus. This is taken both as a sign of increase of the thymus activity and as a support of an autoimmunologic event in ulcerative colitis. These germinal centers appeared in young patients who had suffered from ulcerative colitis for many years.Four cases (including the three with germinal centers) showed a severe, chronic, productive sinus catarrh of the intrathymic lymph nodes, which is probably related to inflammatory thymus processes.