Monozygotic twin boys with fragile X syndrome underwent thorough genetic, psychiatric, neurological, and language evaluations at 10 years of age. They both demonstrated physical features, speech and language difficulties, social problems, and attentional deficits that characterize the behavioural phenotype of fragile X syndrome. Despite identical genetic constitutions, there were important developmental and behavioural heterogeneities. Twin A showed less social interaction and symbolic play and more speech and language dysfunction than twin B. Twin A also had significantly larger caudate volumes. It is suggested that the Xq27.3 anomaly may not be sufficient to account for all the behavioural phenotypic and neuroanatomical features of fragile X syndrome.