Defective control of the alternative route of the complement system is an important cause of the non-diarrhoea haemolytic uraemic syndrome (HUS). On the endothelial surface, mutations in HF1, MCP and IF predispose to development ofHUS. Uncontrolled complement activation on the surface of endothelial cells will damage these cells extensively. Plasmapheresis can be an effective, although temporary, treatment for mutations in HF1 and IF. HUS frequently recurs after renal transplantation in patients with HF1 or IF mutations but not in patients with a mutation of MCP. These genetic defects can be detected by routine diagnostics.