Scalp hair patterns have been examined in 1901 healthy subjects and 1789 severely subnormal patients. Patients with Down's syndrome had a highly significant excess of midline occipital whorls and a deficit of right-sided occipital whorls. Five out of 44 patients with microcephaly had a distinct 'up-sweep' of the frontal hair, a much lower proportion than found previously. Patients with unspecified mental subnormality had a highly significant deficit of multiple occipital whorls. Cutis verticis gyrata was noted incidentally in 15 subnormal patients, 13 of whom were males. Observation of hair patterns in individual patients with mental subnormality is of theoretical interest but is unlikely to be of great practical value.