Among the multiple congenital anomalies (MCA) syndromes, the Noonan syndrome (NS) is a cardiofacial syndrome in which affected individuals may be short and mildly mentally retarded. Autosomal dominant inheritance of Noonan syndrome with variable expressivity has been documented in many families. Genetic heterogeneity has been postulated in Noonan syndrome because of the wide phenotypic variability, the relatively high incidence, and the occasional recurrence in sibs with apparently normal parents. Clinical variability is usual in autosomal dominant disorders, and mildly affected individuals may be difficult to recognize as gene carriers. Thus, a family with two or more affected children may simulate autosomal recessive inheritance. We have studied serial and family photographs of NS individuals in order to assess the likelihood of gene carriers' being missed in genetic studies. We have confirmed wide clinical variability within families, and more importantly, we have documented marked change of phenotype with age from the newborn period, infancy, childhood, and adolescence to adulthood. Manifestations in adults may be subtle and some without a known heart defect or other medically significant problems may have been considered normal in the past. Our study, while not ruling out causal heterogeneity, suggests that the change of phenotype with age may have been falsely perceived as clinical heterogeneity. A particular and subtle phenotype must be searched for in parents of affected children.