A cranial orthosis is contoured to match the curvature of the fronto-temporal, parietal and occipital areas of an infant's cranial vault to provide protection against the acquisition of postural cranial deformities as a result of the infant's sleeping in the supine position. The orthosis is designed to be of universal fit, as determined by the infant's fronto-occipital head circumference (FOC) measurement. Moreover, the interior dimensions of the orthosis can be enlarged to accommodate growth of the infant's head without requiring replacement. The orthosis is a molded plastic appliance in the form of a shell, headband or helmet having interior surfaces that are smoothly contoured to conform in shape to the surface curvature of the occipital, temporal and parietal areas of a healthy human infant having normal cranium size, shape and symmetry. The cavity is sized to provide a close, non-interfering fit of the conformed interior surfaces in facing relation to the occipital, fronto-temporal and parietal areas of the infant's cranial vault, thereby allowing the the infant's head weight forces to spread substantially uniformly across one or more of the conformed interior surfaces while the infant is resting on a sleep surface in the supine position.