Criteria for the classification of hypersensitivity vasculitis were developed by comparing 93 patients who had this disease with 714 control patients with other forms of vasculitis. For the traditional format classification, 5 criteria were selected: age greater than 16 at disease onset, history of taking a medication at onset that may have been a precipitating factor, the presence of palpable purpura, the presence of maculopapular rash, and a biopsy demonstrating granulocytes around an arteriole or venule. The presence of 3 or more of these 5 criteria was associated with a sensitivity of 71.0% and a specificity of 83.9%. A classification tree was also constructed. The criteria appearing in the tree structure were the same as for the traditional format, except there were 2 pathology criteria: one required the presence of granulocytes in the wall of an arteriole or venule, and the other required the presence of eosinophils in the inflammatory exudate. The classification tree was associated with a sensitivity of 78.5% and a specificity of 78.7%.