Eculizumab is the therapy of choice for patients with atypical hemolytic uremic syndrome (aHUS). Dosing recommendations stem from two trials: one retrospective trial (19 children and 5 infants) and one prospective trial (22 patients and 5 infants). This case report highlights the need for more precise dosing recommendations in children, particularly in infants, and for smaller vials of the medication to facilitate more precise dosing. Such changes would ensure that adverse events are minimized and that the children with aHUS who are treated with eculizumab experience an optimal clinical response.