We describe 2 cases of idiopathic retroperitoneal fibrosis presenting with back pain. Additional features included weight loss, constitutional symptoms and elevated erythrocyte sedimentation rate (ESR). One patient had sacroiliitis and was HLA-B27 positive. A further finding was obstruction of the superior mesenteric artery which has not been previously described in retroperitoneal fibrosis. The importance of considering idiopathic retroperitoneal fibrosis in the differential diagnosis of back pain has not been emphasized in the rheumatological literature.