Marfan syndrome is very rarely diagnosed in the neonate, and specific pathological and clinical findings are described in this age; cardiac involvement occurs very often, carrying severe prognosis in most patients. In the presented case the diagnosis was suspected according to clinical findings; bidimensional echocardiography showed that the most important lesion was tricuspid dysplasia and prolapse; Doppler echocardiography has contributed to explain the clinical severity, showing regurgitation of all cardiac valves and a pattern of functional tricuspid atresia. Autoptic examination has later confirmed all these findings.