Here we report an adolescent male with Cushing's disease secondary to a pituitary adenoma who was admitted with cholestatic hepatitis. A detailed work-up for hepatitis was negative except for a novel heterozygous ABCB4 gene mutation encoding multidrug resistance type III (MDR3) protein. Upon resection of the pituitary adenoma, the hepatic biochemical abnormalities gradually improved. This case highlights that heterozygous ABCB4 defect may represent a genetic predisposition for nongenetic factors such as hormones or other metabolites to decrease normal MDR3 allele expression. It is known to act as a modifier gene and possibly played a role in the development of cholestatic hepatitis and cholelithiasis, in the presence of excess cortisol secondary to Cushing's disease in our patient.