During the past decade cyclosporine has played a vital role in pediatric organ transplantation. It exerts a modulatory effect at many sites in the immune response. In young children cyclosporine clearance is significantly increased. Moreover, children are frequently on other drugs which may affect cyclosporine metabolism. These factors have to be taken into account for planning dosing regimens in children. Therapeutic monitoring has added to the efficacy and safety of cyclosporine use, particularly in those situations such as pediatric transplantation where metabolism is appreciably altered.