Criteria for the classification of polyarteritis nodosa were developed by comparing 118 patients who had this disease with 689 control patients who had other forms of vasculitis. For the traditional format classification, 10 criteria were selected: weight loss greater than or equal to 4 kg, livedo reticularis, testicular pain or tenderness, myalgias, mononeuropathy or polyneuropathy, diastolic blood pressure greater than 90 mm Hg, elevated blood urea nitrogen or serum creatinine levels, presence of hepatitis B reactants in serum, arteriographic abnormality, and presence of granulocyte or mixed leukocyte infiltrate in an arterial wall on biopsy. The presence of 3 or more of these 10 criteria was associated with a sensitivity of 82.2% and specificity of 86.6%. A classification tree was also constructed, with 6 criteria being selected. Three of these, angiographic abnormality, biopsy-proven granulocyte or mixed leukocyte infiltrate in arterial wall, and neuropathy, were criteria used in the traditional format. The other 3 criteria used in the tree format included the patient's sex, weight loss greater than 6.5 kg, and elevated serum aspartate aminotransferase or alanine aminotransferase levels above the range of normal. The classification tree yielded a sensitivity of 87.3% and a specificity of 89.3%.