It has been suggested by several authors that the prevalence of mitral valve prolapse and aortic root dilation is increased in individuals with the fragile X syndrome. The prevalence of these abnormalities in young children with fragile X has not been established. Sixteen boys and 1 girl 13 years or younger were studied. Only 1 child (6%) had the typical examination findings of mitral valve prolapse, although his echocardiogram was normal. From this study it appears that cardiac abnormalities in fragile X syndrome develop in late childhood and adolescence. It seems important to follow these children carefully so that these cardiac abnormalities can be documented when they do develop.