Atypical hemolytic uremic syndrome is rare condition. Orphan or rare diseases are chronic life-threatening and require specific means for their treatment (orphan drugs). In many cases the early diagnosis and treatment of orphan disease helps to avoid serious complications and lethal outcome. Clinical case. The patient, 6 years old,presented to the clinic with complaints of fever, vomiting, diarrhea up to 10 times a day. He was treated in outpatient department for 5 days, received symptomatic therapy, was not observed by pediatrician. In connection with the deterioration of condition patient was presented to inpatient infectious department of Children's Infectious City Hospital. The child had severe condition and changes in laboratory parameters (thrombocytopenia, increased levels of urea and creatinine) and was referred to intensive care department. Against the background of treatment, the patient had a persistent thrombotic microangiopathy, plasma resistance, kidney failure , and therefore it was diagnosed hemolytic uremic syndrome. Before starting of plasma therapy, we studied of ADAMTS-13 activity was for verify the diagnosis. The activity of ADAMTS-13 metalloproteinase in the blood plasma was 47% of the level of ADAMTS-13 activity in the control plasma, which was confirmation of the presence of a rare genetic disease in this child. The child received treatment with eculizumab. Conclusion. The presented clinical case shows the difficulties of diagnosis and treatment of orphan diseases. The early diagnosis and correct therapy are necessary for the successful management of children with hemolytic uremic syndrome. Key words: thrombotic microangiopathy, atypical hemolytic uremic syndrome, complement system, children