Clarithromycin is a macrolide antibiotic similar in structure to erythromycin, but suggested to have fewer drug interactions. Although a pharmacokinetic interaction between clarithromycin and cyclosporine was recently reported, its magnitude and mechanism have not been explored. A 43-year-old renal transplant recipient receiving cyclosporine was treated with clarithromycin because of pneumonia. A cyclosporine pharmacokinetic study was performed 8 days after the initiation of the clarithromycin and 14 days after stopping the drug. Clarithromycin coadministration caused an approximately 2-fold increase in the area under the whole blood concentration versus time curve of cyclosporine. The oral clearance of cyclosporine was halved by clarithromycin, but the terminal elimination rate constant decreased only 15% and mean residence time 20%. These observations suggest that clarithromycin inhibits not only the hepatic metabolism but also the intestinal metabolism of cyclosporine. Caution is advised when administering the two drugs concurrently, and additional studies are necessary to elucidate the mechanism of this interaction.