Tightly curled ‘frizzy’ hair is a pathognomonic sign of giant axonal degeneration (GAD). The present study compares the morphological structure of the scalp hair of a GAD patient with that of her parents and first-degree relatives with the aid of scanning electron microscopy. The comparison included plaited hair of two age-matched controls, in order to exclude mechanical plaiting artifacts. Trichorrhexis nodosa and fringing of the cut ends were exclusively found in the patient’s hair. Longitudinal grooving was also frequently seen in hair of normal persons. Assay of carbon (C), sulfur (S) and nitrogen (N) contents of the patient’s hair was normal, but the S:N ratio was significantly reduced, as compared with her relatives. However, comparison with a control group of non-related healthy volunteers showed no dif