Mutations in factor H (HF1) have been reported in a consistent number of diarrhoea-negative, non-Shiga toxin-associated cases of haemolytic uraemic syndrome (DHUS). However, most patients with D-HUS have no HF1 mutations, despite decreased serum concentrations of C3. Our aim, therefore, was to assess whether genetic abnormalities in other complement regulatory proteins are involved. We screened genes that encode the complement regulatory proteins—ie, factor H related 5, complement receptor 1, and membrane cofactor protein (MCP)—by PCRsingle-strand conformation polymorphism (PCR-SSCP) and by direct sequencing, in 25 consecutive patients with D-HUS, an abnormal complement profile, and no HF1 mutation, from our International Registry of Recurrent and Familial HUS/TTP (HUS/thrombotic thrombocytopenic purpura). We identified a heterozygous mutation in MCP, a surface-bound complement regulator, in two patients with a familial history of HUS. The mutation causes a change in three aminoacids at position 233–35 and insertion of a premature stop-codon, which results in loss of the transmembrane domain of the protein and severely reduced cell-surface expression of MCP. Results of previous studies on HF1 indicate an association between HF1 deficiency and D-HUS. Our findings of an MCP mutation in two related patients suggest that impaired regulation of complement activation might be a factor in the pathogenesis of genetic forms of HUS. MCP could be a second putative candidate gene for D-HUS. The protein is highly expressed in the kidney and plays a major part in regulation of glomerular C3 activation. We propose, therefore, that reduced expression of MCP in response to complement-activating stimuli could prevent restriction of complement deposition on glomerular endothelial cells, leading to microvascular cell damage and tissue injury.