The Shiga toxin associated (Stx) hemolytic uremic syndrome (HUS) is an important cause of acute renal failure (ARF) and the most common cause of thrombotic microangiopathy (TMA) in pediatrics. Primary atypical HUS (aHUS) is a rare disease due to a genetic defect in the alternative complement pathway. Both diseases may share clinical and laboratory elements, making differential diagnosis difficult, such as the presence of diarrhea in aHUS or complement alterations in HUS-Stx. The treatment and prognosis of both diseases is completely different. We report a 15-year-old male with severe HUS. After a self-limited diarrheal syndrome, he had a severe TMA and ARF, requiring renal replacement therapy. An extensive etiological study was carried out, ruling out the main causes of TMA. Alterations in complement factors were observed. Finally, the diagnosis of HUS-Stx was established. The patient evolved favorably with recovery of renal function.