The term “hypersensitivity vasculitis” has been associated with much confusion ever since it was incorporated into the first vasculitis classification scheme in the early 1950s. The condition’s name derives from the fact that by the 1950s, both human and animal models of hypersensitivity to foreign antigens had been shown to cause small-vessel vasculitis involving the kidneys, lungs, and other organs besides the skin. Consequently, even microscopic polyangiitis (see Chapter 33), a disorder that commonly affects internal organs as well as the skin and is often associated with antineutrophil cytoplasmic antibodies (ANCAs), was grouped initially under the heading of hypersensitivity vasculitis. Because of the confusion surrounding its name, many clinicians have suggested that hypersensitivity vasculitis be replaced, but no entirely suitable alternative has been found. Terms used synonymously with hypersensitivity vasculitis have included leukocytoclastic vasculitis, cutaneous leukocytoclastic angiitis, and cutaneous small-vessel vasculitis, among others. In evaluating patients with small-vessel vasculitis of the skin, it is critical to remember that skin findings may only herald an underlying disorder involving other organs, as well. Extracutaneous involvement, which mandates reconsideration of the diagnosis, must be excluded with appropriate tests.