Nine patients with the loin-pain/haematuria syndrome are described. The previously reported clinical features of severe recurrent loin pain and tenderness in young women, the relationship of symptoms to use of oral contraceptives, and the demonstration of intrarenal vascular abnormalities by angiography are confirmed. Histology revealed minor non-specific abnormalities, and electron microscopy showed fibrin in the afferent arteriole and in glomerular capillaries of one patient only. No diagnostic changes were found on renography. In all patients except one the heparin-thrombin clotting-time was abnormal, suggesting increased platelet activity or release and providing further evidence of a vascular disorder.