Paroxysmal nocturnal hemoglobinuria (PNH) is characterized by intravascular hemolysis, effectively controlled with eculizumab, a humanized monoclonal antibody that binds complement protein C5. The residual functional activity of complement protein 5 (C5) can be screened using a CH50 assay, which is sensitive to the reduction, absence and/or inactivity of any components of the classical and terminal complement pathway. Little data exist on complement blockade during treatment. From 2010 to 2012, clinical data, hemolysis biomarkers, complement assessment and free eculizumab circulating levels were systematically measured immediately prior to every single injection given to 22 patients with hemolytic PNH while receiving eculizumab therapy. During the study, six patients received at least one red blood cell transfusion. Lack of detectable CH50 activity (defined by CH50 ≤10% of normal values) was found in 184 samples (51%) and was significantly associated with lower lactate deshydrogenase (LDH) levels (p=0.002). Low levels of circulating free eculizumab (<50 µg/ml) correlated with detectable CH50 activity (CH50>10%) (p=0.004), elevated bilirubin levels (p<0.0001), and the need for transfusions (p=0.034). This study suggests that both CH50 activity and circulating free eculizumab levels may help physicians to manage PNH patients receiving eculizumab.