Thrombotic microangiopathy (TMA) is a pathohistological entity characterized by the formation of thrombi in microcirculation, with clinically defined consequent microangiopathic hemolytic anemia (MAHA) and thrombocytopenia with damage to target organs, most often acute kidney injury (AKI). Determining the etiology can be complex. Thrombotic thrombocytopenic purpura (TTP) is caused by reduced activity of the protease ADAMTS13. Most other forms, so-called secondary TMA, are etiologically linked to some pathogenic process in the body, most often a hypertensive crisis. Even in these conditions, a disorder of the complement system is possible, which is the main etiological feature of primary atypical hemolytic uremic syndrome (aHUS). This is why serological analysis of the complement system, and in some cases its genetic analysis, is important. We present the characteristics of ten patients in whom laboratory and/or pathohistological elements of TMA were present, and in whom both serological and gene analysis of the complement system were performed. Genetic pathogenic or likely pathogenic variants associated with the diagnosis of primary aHUS were found in three patients. All of them had MAHA and AKI and pathohistologically proven TMA. In another group of three patients, TMA was found on kidney biopsy, but all other results (including gene analysis of complement components) were unremarkable. These were patients who presented with hypertensive crisis, and the testing was done for the purpose of pretransplant workup. In the remaining four patients, genetic risk variants for the development of aHUS were found. All these patients (except one) had MAHA + AKI. In two patients no TMA was found in the kidney tissue – but the biopsy was done one month after the acute illness. Gene analysis of the complement system is recommended if there is no clear secondary cause of TMA or if it is important for determining therapy, or for kidney transplantation.