Several recent studies have established an association between abnormalities of complement factor H (FH) and the development of hemolytic uremic syndrome (HUS). To identify the relative importance of mutations in FH as a cause of HUS, we have undertaken mutation screening of the FH gene in 19 familial and 31 sporadic patients with FH. Mutations were found in two familial and three sporadic patients, and these clustered in exons 18-20, a domain important for host recognition. Moreover, this study demonstrates that familial HUS is likely to be a heterogeneous condition.