Four patients with the phenotypic characteristics of Noonan's syndrome presented in infancy with a suspected cardiac defect. The cardiac segmental connections were normal. All four were diagnosed to have valvar pulmonary stenosis by cross-sectional echocardiography, the valve being typically thickened and dysplastic as expected for the syndrome. In addition, all had hypertrophic cardiomyopathy and evidence for important abnormalities of the other cardiac valves. Two of the four patients died following cardiac surgery. Of the other two, one child died during interventional catheterization and another died while awaiting cardiac surgery. The cardiac abnormalities, confirmed at postmortem, were dysplastic pulmonary valve with stenosis (four patients), dysplastic aortic valve (three patients), supravalvar aortic stenosis (one patient), small and dysplastic tricuspid and mitral valves (four patients), hypertrophic obstructive cardiomyopathy affecting both ventricles, with asymmetric septal hypertrophy (four patients), atrial septal defect within the oval fossa (one patient). In addition to the usually described abnormalities of the pulmonary valve and ventricular myocardium, the patients described showed a wide spectrum of valvar dysplasia which has received little attention in the past. If these patients do indeed have Noonan's syndrome, and the evidence supports this contention, then these findings support the hypothesis of a more generalized disorder in the development of the ventricles and valves in the syndrome, which has a variable expression. They also stress the need to look carefully for abnormalities of valves other than the pulmonary valve in patients who present with symptoms in infanscy.