Mass spectrometry (MS) with chemical ionization was found to be a useful tool for characterizing epidermal cholesterol esters (CE) and wax esters (WE), and was applied to analyse them in the epidermis of fetal, adult and keloidal human skin. Increases in the concentrations of CE and WE had occurred by 20 weeks of gestation in association with increasing concentrations of ceramides and cholesterol sulphate. Although adult epidermis contained them in similar concentrations at 25-28 weeks of gestation (13-14% of the total less polar lipids), the molecular species were shown to differ between fetal and adult epidermis by MS. Cholesterol oleate comprised 45% of the total molecular species in adult CE, whereas saturated fatty acids with chain lengths of 15-19 were abundant in fetal CE. Also, unsaturated and shorter chain WE were abundant in adult epidermis in comparison with that in the fetal epidermis, the total chain lengths of fetal and adult WE being 34-42 with a mean chain length of 38.6, and 27-41 with a mean chain length of 34.0, respectively, and the unsaturated molecules in fetal and adult WE amounting to 29% and 45%, respectively. Also, CE and WE in the epidermis of keloidal skin were reduced to 67% and 20% of the levels in normal skin, respectively. Thus, epidermal CE and WE were shown to carry eligible structures for construction of the hydrophobic barrier at the skin and amniotic fluid interface as well as that at the skin and air interface.