SummaryOne thousand, nine hundred and thirty apparently normal newborn infants were examined for the presence of the third fontanelle. In addition other newborn infants with abnormalities were also screened for the same defect.The incidence of the third fontanelle was high, in this study, among the normals, compared with past studies. The incidence seemed highest in the group weighing 2 001 to 2 500 g.In the abnormal group, the incidence was very significantly raised in infants suffering from the congenital rubella syndrome or Down's syndrome; it was raised but not significantly in infants presenting with various other abnormalities.The presence of the third fontanelle should alert the examiner to the possibility of other defects in the infant.