Renal dysfunction is a frequent consequence of cyclosporine administration, even when cyclosporine blood concentrations are maintained within proper therapeutic levels. As dermatologie indications for this compound expand, it is important for clinicians to understand the relevant pharmacology and physiology that underlie cyclosporine-induced renal dysfunction. This article reviews the clinical pharmacology of cyclosporine, the principles of renal function testing, and the clinical manifestations of this drug's effects on the kidney. Sudden declines in kidney function, long-term cyclosporine nephrotoxicity, hemolytic-uremic syndrome, and hypertension are discussed from a clinical and experimental standpoint. Finally, practical suggestions are provided to aid the dermatologist in managing cyclosporine-treated patients.