In the current study, the authors examined whether the fatigue level of children diagnosed with cri du chat syndrome was associated with the expression of autistic symptoms. Sixty-nine children with cri du chat syndrome were compared with 47 children with moderate to severe intellectual disabilities who did not differ on intellectual severity. Participants were assessed using the Infant Sleep Questionnaire ( J. M. B. Morrell, 1999 ) for fatigue-level rating and the Childhood Autism Rating Scale ( E. Schopler, R. J. Reichler, & B. R. Renner, 1988 ) for autism-level rating. In support of the authors' hypothesis, results indicated that children who exhibited high levels of fatigue were more likely to express high levels of autistic symptoms. Contrary to the authors' hypothesis, children in the comparison group who exhibited high levels of fatigue conferred the greatest vulnerability to the expression of autistic symptoms.