Acute graft rejection is still a major cause of morbidity after orthotopic liver transplantation, and its diagnosis necessitates an invasive liver biopsy. Our aim has been to determine whether changes in individual serum bile acid levels after engraftment are sensitive, specific and reliable indicators of graft function and whether these changes can antedate other biochemical indicators of hepatic allograft rejection. Individual bile acids in 200 serum samples taken serially from eight adult liver transplant patients were measured. Patients with biopsy-confirmed graft dysfunction due to rejection or nonrejection causes (n = 6 episodes) had significantly higher serum concentrations of glycocholate plus glycochenodeoxycholate and taurocholate/taurochenodeoxycholate ratios than did noncomplicated grafts (n = 3). These changes antedated any other conventional biochemical parameters by at least 48 hr and were 100% sensitive and specific. None of the conventional liver function tests could match this. Acute rejection episodes (n = 3) were then compared with nonrejection causes of graft dysfunction (n = 3). In acute rejection we noted a significant increase in the concentration of glycodeoxycholate plus deoxycholate and a significant decrease in the cholate/chenodeoxycholate ratio compared with that in nonrejection graft malfunction. Both of these changes antedated any other biochemical parameters by 24 hr. In conclusion, individual serum bile acid assays, after orthotopic liver transplantation, can detect graft dysfunction resulting from any cause at an earlier time than routine biochemical tests, and they are sensitive, specific and reliable for early detection of graft dysfunction. In addition, acute rejection can be distinguished from other causes of graft dysfunction.