Goniophotometry has enabled measurement of the angle that the surface of the cuticle (the scales) of a human hair fiber makes with the axis of the hair shaft. This measuring technique has been used to obtain the change of this scale angle with extension of a hair fiber under fixed conditions of temperature and relative humidity. Based on a simple model of overlapping scales of the hair cuticle, analysis shows that, for hair fibers extended to strains above 10% at 35% relative humidity and at 35 degrees C, overlapping scales become progressively detached from each other. This scale detachment has been suggested to result from the mechanical failure of the endocuticle layer in the scale structure. This endocuticle layer is low in disulfide cross-linking, which would result in a lowered stiffness and greater extensibility, as indicated by the higher swelling of the layer in water as against the highly cross-linked exocuticle layers. The greater extensibility of the endocuticle would also explain the greater distortion of this layer under stress, but it would not follow that endocuticular failure under stress should result. An alternative mechanism of failure of adhesion between overlapping scales in the cuticle is suggested, based on the involvement of the hydrophobic upper-beta-layer with its surface of 18-methyleicosanoic acid (18-MEA), which may provide mobility and a reduction in adhesion between scales. This potential failure of the cementing of the overlapping scale structure due to the 18-MEA is discussed, with particular reference to the standard permanent setting procedure of human hair.