Endogenous 13- cis- and all- trans-retinoic acids have been quantitated in human serum using a solvent extraction procedure followed by isocratic reversed phase high performance liquid chromatography and UV detection. In healthy adults, after an overnight fasting period, the concentrations of 13- cis- and all- trans-retinoic acids yielded 5.3 ± 2.43 nmol/l and 11.8 ± 3.3 nmol/l, respectively (mean ±SD). The method has been successfully applied to the analysis of both isomers in serum from patients with idiopathic skeletal hyperostosis in whom, the 13- cis- as well as all- trans-retinoic acid levels were raised as compared to the control group.