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This study was designed to explore the behavioral phenotype of autism in a group of young children with fragile X syndrome (FXS). Twenty-four children with FXS, ages 21 to 48 months, were compared with two well-matched groups: 27 children with autism (AD) and 23 children with other developmental delays (DD), on two standardized autism instruments, as well as on measures of development and adaptive behavior. Two FXS subgroups emerged. One subgroup (n = 16) did not meet study criteria for autism. Their profiles on the autism instruments and the developmental instruments were virtually identical to the other DD group. The other FXS subgroup (n = 8, or 33% of the total FXS group) met study criteria for autism. Their profiles on the autism instruments were virtually identical to the group with autism. The finding of two FXS subgroups raises a hypothesis of additional genetic influences in the FXS autism group, warranting further genetic studies.