A few cases of severe rhabdomyolysis have been reported in heart transplant recipients treated simultaneously with ciclosporin (CS) and the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor lovastatin. When measured, plasma lovastatin HMG-CoA reductase inhibitor concentrations in these patients were higher than expected. This prompted us to study the plasma concentration profiles of simvastatin HMG-CoA reductase inhibitory activity after a single dose of simvastatin in kidney transplant recipients. Five patients treated with CS, azathioprine and prednisolone (CS patients) were compared to 5 patients treated with azathioprine and prednisolone (non-CS patients). The concentration curves had similar shapes but the mean area under the curve/24 h was almost 3 times higher (p = 0.047) and the mean peak concentration was twice as high in CS patients (p = 0.028). These results suggest a difference in the disposition of simvastatin in CS patients as compared to non-CS patients. Simvastatin should be administered in a reduced dosage to CS patients.