Paroxysmal nocturnal hemoglobinuria is a rare non malignant clonal disease, caused by an acquired somatic mutation in a variable number of hematopoietic stem cells, whose consequence is an abnormal sensitivity of blood cells to complement-mediated lysis. It manifests as intravascular hemolytic anemia, a variable degree of bone marrow insufficiency and high thrombotic risk. The effective management of the disease is based on an adequate diagnosis and clinical and laboratory follow-up. Flow cytometry is the method of choice for diagnosis and monitoring of the patient. The only curative treatment is hematopoietic stem cell transplantation. Eculizumab is the first specific treatment approved for this disease. We present the case of a patient diagnosed with paroxysmal nocturnal hemoglobinuria, wo has recently started treatment with eculizumab. We show the clinical and analytical evolution during the first months of treatment as an assessment of is benefit.