Although it has long been recognized that hyperbilirubinemia is enhanced in patients with impaired renal function, a clear correlation between the levels of bilirubin and creatinine in serum has never been established. We have studied this relationship in 13 bone marrow transplant recipients who had episodes of combined conjugated hyperbilirubinemia and renal impairment. Most of the patients had graft-vs-host disease and various degrees of hepatic cholestasis as evidenced by histologic examination results and abnormal liver function tests. Serial serum specimens obtained during this period showed a good correlation between the two analytes, with a mean (+/- SD) correlation coefficient of .86 +/- .10. The slopes of the regression lines, represented as bilirubin-creatinine ratios, varied widely (0.2 to 3.6), reflecting the wide variation in the degree of hyperbilirubinemia attributable to renal impairment.