We present a new case of irreversible hair matting, a rare, but important and alarming, acquired hair disorder. This case was investigated to test the causal hypothesis proposed in 1984. At the same time four other specimens of matted hair from patients and tangles from normal individuals were also examined. Light and electron microscopy showed dramatic permanent twisting and bending of the hairs through 180 degrees. There was marked variation in the fibre width of hairs from matted samples, with some longitudinally split along a considerable length. This bending and entanglement of hairs of varying widths (felting) seemed to be the main reason for the hairs becoming so dramatically knotted together in four of the cases. In only one case was there any evidence of a viscous fluid binding the hair together. This finding suggests that there may be two different mechanisms involved in hair matting.