Cyclophosphamide is a potent anti-inflammatory and immunosuppressive cytostatic and cytotoxic drug used for such diverse medical problems as neoplasia, tissue transplantation, and inflammatory disease of uncertain cause. The drug is inactive until converted to active metabolites in the liver. The serum half-life in humans is about 6.5 hours, and both metabolism and toxicity vary with body surface area. Cyclophosphamide treatment kills both rapidly proliferating cells and resting lymphoid cells. This leads to a reduction of circulating small lymphocytes as well as impaired humoral and cellular immune responses. Infectious complications may result. Toxicity is also particularly evident in the urologic system where sterile hemorrhagic cystitis is a common problem. Other reactions include infertility in both sexes, pneumonitis after treatment, and inappropriate secretion of antidiuretic hormone. Guidelines should be carefully followed in prescribing immunosuppressive drugs.