Compulsive, self-injurious, and autistic behaviors were examined in 31 boys and 29 girls with fragile X syndrome aged 5 to 20 years. Self-injurious behavior occurred in 58% of boys and 17% of girls, whereas compulsive behavior occurred in 72% of boys and 55% of girls and did not appear to be associated with self-injurious behavior. Fifty percent of boys and 20% of girls met diagnostic criteria for autism on the ADOS-G. Girls who showed compulsive behavior had lower levels of FMRP than girls who did not show compulsive behavior, and boys with autistic symptoms had lowered levels of cortisol. Taken together, these data suggest that autistic and compulsive behaviors are highly prevalent in fragile X syndrome and that lowered levels of FMRP and cortisol may be biological markers for these behaviors.