Atypical hemolytic uremic syndrome is associated with mutations in the complement proteins factor H, factor I, factor B, C3, or membrane cofactor protein in about 50% of patients. The evolution and prognosis of the disease in patients carrying mutations in factor H is particularly poor, and renal transplantation most often fails because of recurrence of the disease in the graft. The risk of rapid loss of renal function in patients with functional mutations in factor H requires that effective treatment be initiated as soon as possible, but identification of these patients relies on genetic studies that are time consuming. We describe a case in which an in vitro hemolytic assay proved useful for rapidly assessing factor H dysfunction and for testing whether this dysfunction could be corrected with fresh frozen plasma. In the context of this case, we summarize recent advances in understanding the molecular mechanisms contributing to atypical hemolytic uremic syndrome, including descriptions of DNA- and protein-based analysis. We conclude that functional analysis of factor H should help rationalize the plasma treatment of patients with atypical hemolytic uremic syndrome.