Although macrolides have been associated with significant pharmacokinetic interactions, clarithromycin is considered to have a low interaction capacity. In this study, six transplant recipients treated with cyclosporin A also received clarithromycin. In all patients, the dose of cyclosporin A had to be reduced by a mean of 33% per day depending on the macrolide dose. Normalization of the dosage parameters began on the fourth day after stopping clarithromycin treatment. Co-administration of cyclosporin A and clarithromycin may lead to increases in whole blood cyclosporin levels, and appropriate dose reductions should be considered.