Fever in a transplant recipient is an important sign of graft rejection or infection. Rarely, fever may result from an immunosuppressive agent used to prevent graft rejection. A case of fever, rigors, arthralgias, and myalgias is reported in a cardiac transplant recipient in whom azathioprine therapy was recently begun. These findings resolved on discontinuation of the azathioprine, recurred on rechallenge, and were most consistent with a hypersensitivity reaction. The clinical spectrum of reported azathioprine hypersensitivity reactions is reviewed. © 1988.