Two familial clusters of systemic vasculitis are described. In one family, microscopic polyangiitis and rapidly progressive glomerulonephritis occurred in HLA-identical siblings; in the second family, 3 second- and fourth-degree related members were affected by Wegener's granulomatosis. Published clusters of systemic vasculitides and Goodpasture's syndrome are reviewed, and, together with the observed families, the evidence for genetic susceptibility and a causative role of environmental factors for these diseases with special emphasis on the HLA system is discussed.