Following an alcohol binge, cyclosporine A (CyA) levels rose by 100% in a 51-year-old transplant recipient treated with CyA. As CyA and ethanol are both metabolized by the cytochrome P-450 enzyme system, ethanol could theoretically interfere with CyA metabolism. Therefore, eight male renal transplant recipients were assessed in a crossover study to determine the effects of acute ethanol ingestion on CyA serum concentrations. CyA serum concentrations did not rise following 50 mL of 100% alcohol. We conclude that heavy alcohol intake may increase CyA levels but that acute moderate intake does not.