The fragile X syndrome, a common X-linked form of mental retardation and autism, affects females as well as males. Previous work has shown that approximately 35% of heterozygotes (women who carry the fragile X gene) demonstrate cognitive impairment. Thirty-two girls, 18 years or younger, who demonstrate the fragile X chromosome were evaluated and compared with 19 sisters who do not demonstrate the fragile X chromosome. Evaluations included a physical examination, behavioral assessment, and intelligence testing. Significant differences (in intellectual, behavioral, and physical features) were seen between the two groups. Twenty-five percent of fragile X-positive girls had an IQ in the mentally retarded range (IQ less than 70) and 28% had an IQ in the borderline range (70 to 84). Prominent ears, shyness, and poor eye contact were significant findings in fragile X-positive girls compared with fragile X-negative girls. Thirty-one percent of the fragile X-positive girls had significant attentional difficulties and most of these girls were successfully treated with stimulant medication. The majority of fragile X-positive girls in this study demonstrated significant behavioral and developmental problems which required identification and appropriate treatment. Pediatricians and health care providers should be aware of the frequency and manner with which fragile X affects females in order to initiate cytogenetic studies and treatment when indicated.