Recently, two new systems have been proposed for the nomenclature of primary vasculitides: the 1990 American College of Rheumatology (ACR) classification criteria and the 1992 Chapel Hill Consensus Conference (CHCC) definitions. We compared these two systems in the same cohort of patients with primary systemic vasculitis. Twenty-four patients were studied and, applying the 1990 ACR criteria, the diagnoses were Wegener's granulomatosis (WG) (15), Churg Strauss syndrome (CSS) (4), polyarteritis nodosa (PAN) (2) and unclassified (3). Using the CHCC definitions, the diagnoses were WG (5), microscopic polyangiitis (MPA) (8), possible MPA (1), PAN (1) and undefined (9). There was concordance in only five patients (all WG). Significant discordance exists between these two criteria sets. Since the ACR criteria set does not include MPA, WG tends to be overdiagnosed. The Chapel Hill definitions are biopsy dependent and surrogate features for the defining histology are required to allow their practical application.