Drug related vasculitis has variously been described as necrotizing hypersensitivity or allergic angiitis or microscopic panarteritis nodosa. We reviewed tissue sections from 30 patients with validated drug hypersensitivity and vasculitis in order to precisely define this entity. No evidence of necrotizing vascular lesions or of fibrinoid associated with necrosis was found. The vascular lesions in all 30 patients involved small arteries, arterioles, capillaries, and venules. The inflammatory infiltrate consisted primarily of mononuclear cells and prominent numbers of eosinophils and was present in all three layers of the involved vessel walls. Clinically the patients developed either localized or systemic vasculitis, which could not be predicted on the basis of the associated drug. The findings of a skin rash, fever, or eosinophilia and the development of symptoms consistent with a hypersensitivity reaction while medication was being taken were all suggestive of the diagnosis of drug related vasculitis.