The concomitant occurrence of retroperitoneal fibrosis (RF) and Riedel's thyroiditis is reported in two patients, one of whom presented with anuria. The etiology is discussed, and it is concluded that immunological mechanisms with arteritis might be of importance. The associations between Riedel's thyroiditis, RF, mediastinal fibrosis, sclerosing cholangitis, retroorbital fibrosis and fibrosis in other organ systems are thought to be different manifestations of a so-called multifocal idiopathic fibrosclerosis. The beneficial effect of glucocorticoids in early stages of the disease is mentioned.