Chronic administration of cyclosporin A may induce cholestasis in a few patients. The purpose of this study was to examine the effect of chronic administration of cyclosporin A on serum bile acid levels, serum bilirubin concentration, and bromosulfophthalein plasmatic fractional clearance. Twenty heart-transplanted patients with normal serum alanine aminotransferase activity receiving cyclosporine A during a mean duration of 33 months (range 7-54) were compared to 20 matched kidney-transplanted patients with normal serum alanine aminotransferase receiving azathioprine for a mean duration of 34 months (range 6-72). As compared to azathioprine-treated patients, patients treated with cyclosporin A had an increase in serum bile acid levels of 32% (P < 0.01), an increase in serum bilirubin concentration of 100% (P < 0.001), and a decrease in bromosulfophthalein plasmatic fractional clearance of 60% (P < 0.001). These results suggest that cyclosporin A induces a decrease in hepatic excretory function in man.