Cyclosporine constitutes a major advance in pharmacologic immunosuppression, the benefit of which is now established for solid organ transplantation and is rapidly emerging for many forms of autoimmune disease. By virtue of its potency and selectivity, there has been a marked reduction in steroid requirement with a concomitant reduction in morbidity and mortality. The undesirable effect of cyclosporine on the kidney may thus be considered within this context. The short-term functional effect observed to some degree in most patients receiving this drug is rapidly reversible, and is unaccompanied by long-term detriment in studies now extending over 6 years. Progressive deterioration still occurs in a small proportion of patients, but may often be reversed by carefully controlled conversion to alternative combination immunosuppression therapy. For each developing application, the ultimate value of cyclosporine must be determined individually in relation to the severity of the disease process. The challenge that now confronts us is to determine the manner in which this agent may be most safely and effectively used.