European guidelines for the assessment and management of atypical HUS were written in 2009. Since then our understanding of this group of diseases has advanced. Evidence is emerging that eculizumab, a monoclonal antibody inhibiting C5 activation, is effective, and potentially superior to current treatment with plasmapheresis. The evidence base for the benefits of plasmapheresis consists of case series and small reports. Before we embark on a change of management policy it is vital that we set up a system for genetic diagnosis, standardised protocols and a means to collect predetermined outcome measures, so that we do not make the same mistakes with assessment of the effectiveness of eculizumab as we did for plasmapheresis.