Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 12.0-17.0,  Atypical Hemolytic Uremic Syndrome Male or female patients from 12 and up to 18 years weighing ≥ 40 kg who have been diagnosed with Atypical Hemolytic-Uremic Syndrome (aHUS). 2. Decrease in platelet count despite at least 4 plasma therapy (PT) treatments in the 1 week immediately prior to screening. 1. Screening platelet count , < 150 x10^9/L and at least 25% lower than the average remission platelet count or 2. If remission counts not available, platelet count at onset of the current aHUS episode must be ≤75x10^9/L and platelet count at screening must be ≤ 100 x 10^9/L despite PT treatment administration of at least 4 PT treatments in the 1 week immediately prior to screening 3. Known complement regulatory protein genetic abnormality 4. Lactate dehydrogenase (LDH) level ≥ ULN unless the patient has been receiving plasma exchange and LDH at the onset of the current aHUS episode was at least the ULN. If LDH is normal at screening, other markers indicative of ongoing hemolysis such as haptoglobin, schistocytes should be evaluated and discussed with Sponsor 5. Creatinine level ≥ ULN for age 6. Female patients of childbearing potential must be practicing an effective, reliable and medically approved contraceptive regimen during the entire duration of the study, including the follow-up period and for up to 5 months following of eculizumab treatment discontinuation. 7. Patient's parents/legal guardian must be willing and able to give written informed consent and patient must be willing to give written informed assent. 8. Able and willing to comply with study procedures  TTP, (defined as activity <5%) from an historical observation (prior to initiation of plasma therapy) or as tested at the screening visit by the central laboratory 2. Malignancy within 5 years of screening. 3. Typical HUS (Shiga toxin +). 4. Known HIV infection. 5. Identified drug exposure-related HUS. 6. Infection-related HUS. 7. HUS related to bone marrow transplant 8. HUS related to vitamin B12 deficiency 9. Renal function status requiring chronic dialysis 10. Patients with a confirmed diagnosis of sepsis 11. Presence or suspicion of active and untreated systemic bacterial infection that, in the opinion of the Investigator confounds an accurate diagnosis of aHUS or impedes the ability to manage the aHUS disease. 12. Pregnancy or lactation. 13. Unresolved meningococcal disease. 14. Known Systemic Lupus Erythematosus (SLE) or antiphospholipid antibody positivity or syndrome. 15. Any medical or psychological condition that, in the opinion of the investigator, could increase the patient's risk by participating in the study or confound the outcome of the study. 16. Patients who have received previous treatment with eculizumab 17. Patients receiving IVIG within 8 weeks or Rituximab therapy within 12 weeks of screening. 18. Patients receiving other immunosuppressive therapies such as steroids, mTOR inhibitors or tacrolimus are excluded unless: [1] part of an established post-transplant anti-rejection regime, [2] patient has confirmed anti-CFH antibody requiring immunosuppressive therapy, and [3] dose of such medications have been unchanged for at least 4 weeks prior to the screening period or [4] patient is experiencing an acute aHUS relapse immediately after transplant 19. Patients receiving Erythrocyte Stimulating Agents (ESAs) unless already on a stable dose for at least 4 weeks prior to the screening period or a washout period of at least 2 weeks from the last dose of ESA therapy. 20. Participation in any other investigational drug trial or exposure to other investigational agent, device, or procedures beginning 4 weeks prior to screening and throughout the entire trial. 21. Hypersensitivity to eculizumab, to murine proteins or to one of the excipients 22. Patients between ages from 12 and up to 18 years weighing < 40 kg