The production of repetitive speech during conversations was studied in people with fragile X syndrome, autistic disorder, or mental retardation not caused by fragile X. Repetitive speech was found to be more prevalent among those with fragile X compared to the control groups, especially within atypical utterances. These results suggest that the repetitive speech seen in individuals with fragile X is not the result of either general developmental delay or undiagnosed autistic disorder, and they support our hypothesis that such speech dysfluency reflects the effects of physiological arousal caused by hypersensitivity to social and sensory stimuli. Our results are interpreted within Perkins' theory of neuropsycholinguistic function.