A method for the determination of conjugated and unconjugated 3β-hydroxy-Δ5-bile acids and related bile acids in human urine and serum has been developed using high-performance liquid chromatography electrospray ionization-tandem mass spectrometry. Calibration curves for the bile acids were linear over the range of 10 2000 pmol/mL, and the detection limit was less than 4 pmol/mL for all bile acids using selected reaction monitoring analysis. The bile acids in urine and serum samples from two patients with severe cholestatic liver disease were measured by this analytical method. Glycine-conjugated 3β-hydroxy-Δ5-bile acid 3-sulfates were determined to be the major bile acids in the urine and serum from patients with a 3β-hydroxy-Δ5-C27-steriod dehydrogenase/isomerase deficiency or dysfunction.