Treatment of idiopathic retroperitoneal fibrosis by immunosuppression. Idiopathic retroperitoneal fibrosis is exceedingly uncommon in childhood and its etiology is uncertain. Support for an immunological basis for the disease is given by a report of a 14-year-old girl with severe retroperitoneal fibrosis causing progressive azotemia in whom azathioprine and prednisolone were used successfully. This case supports the efficacy of immunotherapy in the treatment of idiopathic retroperitoneal fibrosis. 