Within one year three of 25 patients with rheumatoid arthritis treated with azathioprine 100 mg daily developed the following adverse reactions less than two weeks after starting treatment: patient one showed fever with chills, rash, and severe liver function abnormalities suggestive of cholestasis; the second patient had fever, nausea, diarrhoea, and moderately raised liver enzymes; the third patient showed very high fever and severe chills. In two patients the drug was rechallenged, with more rapidly arising and more severe symptoms. In one case raised liver enzymes persisted until seven months after discontinuation of azathioprine. Hypersensitivity reactions and hepatotoxicity of azathioprine are discussed.