Objective: This study attempted to examine the relationship between month and season of birth and risk for autism spectrum disorders. Method: The cohort included all Jewish individuals born in Israel over 5 consecutive years (N=311,169) and assessed by the Israeli Draft Board as part of the mandatory assessment of eligibility for military service conducted at age 17. The outcome of autism spectrum disorders was ascertained from the Draft Board Medical Registry, which contains information about medical and psychiatric disorders for this population of adolescents. Results: There was no association between month or season of birth and the prevalence of autistic spectrum disorders. Conclusions: The findings from this historical, population-based cohort study do not support an association between season of birth and autistic spectrum disorders.