BACKGROUND: Atypical hemolytic uremic syndrome (aHUS) is a devastating form of renal thrombotic microangiopathy. Despite plasma exchange, the standard treatment of aHUS for decades, the renal prognosis for patients with aHUS has remained poor. We assessed the off-trial use of eculizumab in adult patients with aHUS affecting the native kidneys. STUDY DESIGN: A retrospective study was conducted. aHUS was defined as the presence of 3 or more of the following: acute kidney injury (serum creatinine >1.4 mg/dL [120 μmol/L]), mechanical hemolytic anemia, thrombocytopenia, and the presence of thrombotic microangiopathy features in a kidney biopsy specimen. Patients who had received 4 or more weekly 900-mg infusions of eculizumab were included. SETTING & PARTICIPANTS: 19 patients were identified through a query sent to all French nephrology centers. OUTCOMES & MEASUREMENTS: Evolution of kidney function, hemolysis, and thrombocytopenia after the initiation of eculizumab therapy. RESULTS: All patients had acute kidney injury (serum creatinine range, 2.2-17.0 mg/dL) and 12 required hemodialysis. Thirteen patients carried a mutation in 1 complement gene and 1 had anti-factor H antibodies. For first-line therapy, 16 patients underwent plasma exchange and 3 patients received eculizumab. Median time between aHUS onset and eculizumab therapy initiation was 6 (range, 1-60) days and median time to platelet count normalization after eculizumab therapy initiation was 6 (range, 2-42) days. At the 3-month follow-up, 4 patients still required dialysis, 8 had non-dialysis-dependent chronic kidney disease, and 7 had normalized kidney function. At last follow-up (range, 4-22 months), 3 patients remained dialysis dependent, 7 had non-dialysis-dependent chronic kidney disease (estimated glomerular filtration rate, 17-55 mL/min/1.73 m(2)), and 9 had normal kidney function. Risks of reaching end-stage renal disease within 3 months and 1 year of aHUS onset were reduced by half in eculizumab-treated patients compared with recent historical controls. LIMITATIONS: Retrospective study and use of historical controls. CONCLUSIONS: Our data indicate that eculizumab improves kidney disease outcome in patients with aHUS.