Bile acid synthetic defects represent a specific category of metabolic liver disease. This article highlights the history and summarizes our analytical approach to the diagnosis and treatment of genetic defects in bile acid synthesis. By the application of mass spectrometry as a screening tool, it is possible to perform rapid diagnosis of potential inborn errors in bile acid synthesis from urinary bile acid analysis. Molecular techniques then afford the identification of specific mutations in genes encoding the enzymes responsible for bile acid synthesis. Using this approach, 6 of the 7 known genetic defects that are causes of progressive cholestatic liver disease, syndromes of fat-soluble vitamin malabsorption, or neurological disease, have been characterized. Bile-acid therapy using oral cholic acid has proven effective in most of these bile acid synthetic defects making early diagnosis crucial to optimum clinical prognosis.