Azathioprine-induced pure red blood cell aplasia.

Pure RBC aplasia developed in two renal transplant recipients who were receiving long-term azathioprine therapy. With substitution of cyclophosphamide therapy for azathioprine, erythroid hyperplasia and reticulocytosis developed at three weeks in one patient and at three months in the other. Selective erythroid toxic reaction is a potential problem that must be considered when anemia develops in patients receiving long-term azathioprine therapy.