BACKGROUND: Atypical hemolytic uremic syndrome (aHUS) is a thrombotic microangiopathy characterized by hemolysis, platelet consumption, and renal injury. Eculizumab, a mAb that blocks complement activity, has been successfully used in aHUS. OBJECTIVES: To optimize eculizumab therapy in aHUS patients by monitoring complement functional tests and markers of disease activity. PATIENTS/METHODS: We studied 18 patients with aHUS (10 males; eight females; age range, 2-40 years) treated with eculizumab to induce and/or maintain disease remission. Patients were followed up for a cumulative observation period of 160 months, during which blood samples were obtained at various time intervals to measure complement activity (Wieslab for the classical, alternative and mannose-binding lectin complement pathways) and the parameters of disease activity (haptoglobin and lactate dehydrogenase serum levels, and platelet count). The intravenous eculizumab doses of 12-33 mg kg(-1) were initially administered every week, with the interval between doses being gradually extended to 2 weeks, 3 weeks and 4 weeks on the basis of strict laboratory and clinical control. RESULTS: Complement activity was normal before eculizumab treatment, regardless of the state of the disease (activity or remission). It was completely suppressed 1 week, 2 weeks and 3 weeks after the last eculizumab infusion (mean values ± standard deviation: 1% ± 1% to 3% ± 5% for both the classical and alternative pathways; P = 0.0001 vs. baseline), and partially suppressed after 4 weeks (22% ± 26% and 16% ± 27%; P = 0.0001 vs. baseline). The increase in the time interval between eculizumab infusions did not change disease activity markers. CONCLUSIONS: Monitoring complement tests can allow a safe reduction in the frequency of eculizumab administration in aHUS while keeping the disease in remission.