Objectives. The purpose of this study was to determine the incidence of cardiologic abnormalities in Noonan syndrome.Background. The incidence of cardiac abnormalities in Noonan syndrome remains unknown, largely because of such difficulties as assembling a substantial cohort, ensuring a correct phenotypic diagnosis and providing accurate definitions of the most frequent abnormalities—pulmonary stenosis and left ventricular hypertrophy.Methods. A cohort of 145 patients was assembled, and before cardiologic assessment two independent geneticists scrutinized the phenotype. The diagnosis was confirmed in 118 patients, and they were studied by two-dimensional and Doppler echocardiography.Results. A dysplastic pulmonary valve was present in eight patients (7%) and was associated with significant stenosis in six (75%) of the eight. Significant stenosis was present in 22 (20%) of 110 patients without dysplasia. Left ventricular hypertrophy was present in 29 patients (25%) without significant pulmonary stenosis. Localized anterior septal hypertrophy was the most common pattern in 12 (41%) of 29 patients. Diffuse hypertrophy involving the entire septum and the free wall was present in nine patients (31%) and was severe (>1.7 cm) in five. Other abnormalities included secundum atrial septal defects (10%).Conclusions. The high incidence of cardiac abnormalities suggests that echocardiographic and Doppler evaluation of patients with the Noonan phenotype is important because it will aid in genetic counseling and in the assessment of the natural history of—and, ultimately, identification of the gene(s) responsible for—Noonan syndrome.