AbstractStudies of hair‐follicle development and scalp‐hair patterning innormal fetuses and children and in those with disorders of early brain development were indicative of the following hypothesis: hair directional slope is secondary to the plane of stretch exerted on the skin by the growth of underlying tissues during the period of downgrowth of the hair follicles, around 10–12 gestational weeks. The posterior parietal hair whorl was interpreted as the focalpoint from which the growth stretch is exerted by the domelike outgrowth of thebrain during the time of hair follicle development. Anomalies such as encephalocele and dicephaly, which must have antedated hair follicle development, showed expected aberrations in scalp patterning. Among patients with primarymicrocephaly 85% had altered scalp hair patterning, indicating an early onsetof the problem in brain development. This included 25% with no parietal whorl, a finding previously noted only in nonhuman primates. Aberrant scalp patterning was also found to be a frequent finding in five established syndromes, included Down syndrome, in each case being compatible with a problem in early brain development. Thus, aberrant scalp‐hair patterning may be utilized as an indicator of altered size and/or shape of the brain prior to 12 weeks of ge