Endothelin-1 (ET-1) has been associated with the development of hypertension in cyclosporine-treated renal transplant recipients. To clarify this association, this observational study compared ET-1 levels in 33 patients taking cyclosporine therapy versus 10 controls. Three hours after cyclosporine administration, ET-1 levels were higher than in the controls, namely 1.81 + 0.99 versus 1.17 + 0.46 pg/mL (P <.04). These results suggest a role of ET-1 in the pathogenesis of post-renal transplantation hypertension in cyclosporine-treated recipients.