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Autism spectrum disorders are a heterogeneous group of disorders that share the presence of two core symptoms: problems in social interaction / communication and the tendency to present restricted interests and repetitive behavior. Over the last years, several epidemiologic studies have been published by different authors in diverse countries, having all of them shown two common characteristics: a global increase in the prevalence rates of autism spectrum disorders, and the existence of a great geographical variability no only between geographical areas, but also within the same geographical areas. At the present manuscript, we analyze some of the most recent prevalence data published in USA and some European countries (including Spain). All of them show different prevalence rates, ranging from 1/59 children with autism spectrum disorders in the USA to 1/806 in Portugal. In a second part, we briefly describe some of the current scientific hypotheses that try to explain this variability.