A patient with mitral stenosis was placed on quinidine for control of atrial fibrillation. Eight days later she developed high fever, chills, and diarrhea. Antibiotic therapy was given, and quinidine was discontinued, with abatement of the fever. Readministration of quinidine resulted in a prompt rise in temperature and confirmed the diagnosis of quinidine fever, a relatively rare allergic manifestation of quinidine toxicity.