The vasculitides constitute a heterogeneous group of diseases characterized by blood vessel inflammation and necrosis with different but frequently overlapping clinical and pathologic manifestations. The incidence of these conditions is frequently controversial. To further investigate the incidence and clinical manifestations of vasculitides, we reviewed the spectrum of these diseases in an unselected population of adults (age > 20 years) from northwestern Spain during a 10-year period. From January 1988 through December 1997, 267 adults were diagnosed as having vasculitis. The overall average annual incidence rate of vasculitis in the region of Lugo, Spain, between 1988 and 1997 for the population older than 20 years was 141.54/million. Primary vasculitis (115.04/million for the population older than 20 years; 81.3%), especially giant cell arteritis (GCA) was the most common group. Small vessel primary vasculitis (hypersensitivity vasculitis and Henoch-Schönlein purpura) was the second most common group. Both GCA and small vessel primary vasculitis had a good outcome. However, although less common, patients with medium and small vessel primary vasculitis, in particular those with polyarteritis nodosa, had a high mortality related to the systemic manifestations of the disease or to the immunosuppressive therapy. Among the group of adults with secondary vasculitis (26.51/million; 18.7%), rheumatic diseases and specifically those occurring in the context of rheumatoid arthritis were the most common group. Patients with secondary vasculitis had clinical or laboratory data that may suggest the presence of an underlying disease. In summary, systemic vasculitides are somewhat more common than previously considered. As in other western countries, GCA constitutes the most common type of vasculitis in northwestern Spain. Better physician awareness may contribute to the progressive increase in the recognition of these conditions.