OBJECTIVES: To uniquely classify children with MPA, describe their demographics, presenting features, initial treatments, and compare with GPA patients. METHODS: The European Medicines Agency (EMA) classification algorithm, applied by computation to categorical data of patients recruited to A Registry for Childhood Vasculitis, censored to November 2015, uniquely distinguished MPA from GPA patients who were classified with adult and pediatric-specific criteria. Descriptive statistics were used for comparisons. RESULTS: 231 (64% female) of 440 patients fulfilled classification criteria for either MPA (n=48) or GPA (n=183); respectively median time-to-diagnosis was 1.6 and 2.1 months, range to 39 and 73 months. Comparing MPA versus GPA patients respectively they were significantly younger (median 11 versus 14 years); constitutional features were equally common; pulmonary manifestations (44% versus 74%) were less frequent and less severe (hemorrhage, oxygen-requiring, pulmonary failure); renal features (76% versus 83%) were similarly frequent but tended towards greater severity (nephrotic-range proteinuria, dialysis-requirement, end-stage disease). Airway/eye involvement was absent among MPA patients as these GPA-defining features preclude an MPA diagnosis within the EMA algorithm. MPA and GPA patients respectively received combination therapy with corticosteroids plus cyclophosphamide (69% and 78%) plus plasmapheresis (19% and 22%). Other treatments in decreasing frequencies from 13% to 3% were rituximab, methotrexate, azathioprine, and mycophenolate mofetil. CONCLUSION: Younger onset age, and perhaps both gastrointestinal manifestations and worse kidney disease seem to characterize children with MPA versus GPA. Delay in diagnosis suggests suboptimal recognition. Compared to adults, initial treatments are comparable, but the complete reversal of female to male prevalence ratios is provocative. This article is protected by copyright. All rights reserved.