Inclusion Criteria:  Adults with an atypical Hemolytic and Uremic Syndrome defined by at least 2 of the following parameters:  Thrombopenia (platelet count < 150 G/L) Mechanical lytic anemia (Hb < 10 g/L, Lactate dehydrogenase > upper limit of normal, undetectable haptoglobin, presence of schistocytes on blood smear) Acute renal failure  Eculizumab treatment  Without project of withdrawal in the next 18 months Started since at least 6 months Administrated with an unchanged administration schedule since at least 3 months (initial episode or relapse) Atypical Hemolytic and Uremic Syndrome in remission Patients who give informed consent.  Exclusion Criteria:  Patients on dialysis. Women treated starting or planning a pregnancy. Patients suffering from a typical or secondary microangiopathic hemolytic anemia (drugs, malignancies, autoimmune disease...) Patients under protection of a judicial authority