The response of retroperitoneal fibrosis to tamoxifen. Although retroperitoneal fibrosis is uncommon and histologically benign, it is a progressive and potentially fatal tumor. As the fibroblasts proliferate, they encase and may obstruct important retroperitoneal structures. Medical therapy in the past has been ineffectual, and since the tumor usually cannot be resected, surgery consists of lysis or bypass of the involved structures. Tamoxifen is effective in the treatment of desmoid tumors, and we report its use in two patients with retroperitoneal fibrosis with excellent results. The simplicity and safety of this treatment make tamoxifen an attractive choice of therapy. 