The systemic vasculitides continue to present the practising clinician with many challenges. The spectrum and severity of disease is broad from life or sight threatening fulminant disease to relatively minor skin disease. There has been over the past five years steady progress in our understanding of pathogenesis but still the precise nature of the triggering event or events remains elusive. The aetiology is clearly multifactorial; among the potential influences on disease expression are ethnicity, genes (HLA and others), sex, and environment (ultraviolet light, infections, toxins, drugs, smoking and surgery). There are sufficient differences even within the limited epidemiological data available to suggest that not all these influences work in the same direction in the various described vasculitis syndromes. This editorial will concentrate on recent developments in the classification, epidemiology, and role of environmental factors in pathogenesis.

The classification of vasculitis remains controversial. Although the American College of Rheumatology (ACR) in 1990 produced classification criteria for the major conditions1 and the Chapel Hill Consensus Conference (CHCC) in 1994 developed definitions,2 there are still a number of difficulties. The ACR criteria perform reasonably well when used to compare one type of systemic vasculitis with another; they perform less well when the clinical syndrome is poorly defined, some patients without vasculitis may fulfil ACR criteria for vasculitis.3 The criteria were of course designed for comparing groups of patients (for example, for therapeutic studies) but not for the diagnosis of individual patients and should only be used for that purpose with caution. The CHCC definitions provide the only working definition of microscopic polyangiitis (MPA). MPA was not included as a diagnostic category by the ACR and hence the CHCC definition is increasingly being used for classification; again this was not the original purpose but is the best available. A revision of …