Trismus is caused by sustained contraction of the jaw-closing muscles. It is rare in the neonate and can be a part of developmental defects.Twenty-four neonates, aged 1 to 12 days, were admitted from 1980 to 1992 because they suffered from trismus. All of them had severe difficulties of sucking and/or swallowing, some (12/24) had repeated episodes of apnoea requiring tracheotomy. Specialized investigations included transcutaneous PO2 and PCO2 monitoring, polygraphic recordings during sleep, 24-hour Holter monitoring, ocular compression test, barium swallow, manometry and 24-hour pH monitoring of the distal esophagus, electromyography of muscles involved in swallowing and several imaging techniques.Twenty-one patients had anomalies of the facies and brain stem dysfunctions. They were classified as Robin sequence (14 patients) with (five patients) or without (nine patients) micrognathia, cleft palate and glossoptosis, as Hanhart syndrome (four patients) and Moebius syndrome (three patients). The last three patients had arthrogryposis (two patients) and Stuve-Wiedeman syndrome. Twenty-two of 24 patients had severe gastroesophageal reflux, 15 of 20 had increased vagal reactivity and five of 24 had anomalies of the central nervous system. Eight patients died during the first year of life.Neonatal trismus is a poor prognostic sign, requiring specialized investigations and frequently resuscitation techniques.