Stereochemical analysis of 12-hydroxy-5,8,10,14-eicosatetraenoic acid derived from the lesional scale of patients with psoriasis is reported. Resolution of the C-12 hydroxyl enantiomers was carried out by high pressure liquid chromatography of their diastereomeric methyl ester dehydroabietyl urethane derivatives. The 'psoriasis derived" compound was shown to be stereochemically distinct from the platelet 12(S)-enantiomer as its derivative co-chromatographed with the 12(R)-diastereomer.