Atypical hemolytic uremic syndrome (aHUS) is mostly linked to defects in the regulation of alternative complement pathway, but a rare form is caused by an inherited defect of cobalamin 1 metabolism. Cobalamin C (cblC) deficiency is an autosomal recessive disorder of vitamin B12 metabolism that results from mutations in methylmalonic aciduria and homocysteinuria (MMACHC). The most severe form of cblC deficiency and the associated high mortality rate are mostly observed in neonates or in infants <6 months of age. Early diagnosis of cblC deficiency leads to early treatment and an improved prognosis. We describe the case of a 6-year-old girl with cblC disorder, who presented with severe multiorgan involvement at the age of 5 months and who was successfully treated with vitamin B12, betaine, coenzyme Q10 and l-carnitene, and who had a new homozygous mutation of MMACHC.