Vasculitis is inflammation of blood vessels, which may also affect the arteries of the kidney. The definition and classification of vasculitides derive from the Chapel Hill nomenclature criteria (CHCC) and American College of Rheumatology (ACR) criteria, respectively. These criteria are not perfect, and adult literature refers to problems with misclassification and a low concordance between the two criteria. The problems associated with defining a child patient according to these criteria are even more apparent, since both sets of criteria have been based on adult data alone and have never been validated in children. When classifying their patients as having polyarteritis, paediatricians do not keep strictly to the CHCC or ACR criteria. A recent series has shown that the microscopic polyarteritis associated with MPO-ANCA and the HBs associated with classic polyarteritis nodosa are rarer in children than in adults. Paediatricians should revise the existing criteria based on registries and international consensus in the field.