Fragile X syndrome is the most common inherited cause of developmental and learning disabilities. Both males and females can be affected with a variety of developmental problems ranging from normal intelligence quotient (IQ) with learning disabilities to severe mental retardation and autism. Diagnosis of this disorder in the infant or young child is especially difficult, since many of the characteristics of the syndrome are not recognizable until later ages. This article reviews the genetics of the disorder, the inheritance patterns in families with fragile X, and the key physical, behavioral, and medical features of fragile X in the very young child. Issues relating to temperament and social development relevant to the fragile X child are discussed.