The use of cyclosporine, a fungal endecapeptide immunosuppressive agent, has greatly improved the outcome of haploidentical transplantation in adults, but less impressively improved the result of parent to child transplantation. The incidence of allograft loss and treated rejection episodes was much greater in pediatric than in adult recipients, and the evidences of nephrotoxicity lessened. Although resistance of the child's immune system to the effects of the drug cannot be excluded, it appears more likely that this relates to the rapid clearance of the agent in the pediatric age group (39.6 mL/min/kg) versus in adults (12.3 mL/min/kg), thereby reducing the area under the serum concentration curve from 765 +/- 593 to 386 +/- 277 ng/mL/hr per mg/kg (mean +/- SD). This effect caused trough serum levels measured by radioimmunoassay to be below the putative threshold. These findings demonstrate the need for higher cyclosporine doses and frequency in pediatric compared with adult patients.