We report on a newborn infant with a typical form of Noonan's syndrome. Because of the presence of a fetal cystic hygroma and normal karyotype the diagnosis was suspected before birth. A progressive hypertrophic cardiomyopathy was also found following fetal echocardiographic scanning. We conclude that in addition to cystic hygroma with normal karyotype, the presence of functional or morphological evidence of myocardial abnormality should be included also as prenatal features of Noonan's syndrome.