Physical features of Prader-Willi syndrome in neonates. A retrospective study of 16 patients was undertaken to identify physical features that may typify neonates with Prader-Willi syndrome. Several features known to be typical of Prader-Willi syndrome in early infancy were confirmed, including hypotonia and genital hypoplasia. A number of features that have not previously been emphasized as characterizing Prader-Willi syndrome were also identified, most notably abnormal cry and, in males, signs of genital hypoplasia but with an apparently normal phallus. Other features included disproportionately large head circumference, disproportionately large anterior fontanelle, mild micrognathia, mild anomalies of the gingivae or alveolar ridges, and changes in the appearance of the skin. Appreciation of these features may assist the pediatrician in recognizing the child with Prader-Willi syndrome during the neonatal period, before the appearance of better-known findings of later onset, such as obesity and acromicria. 