Twin studies have shown a strong genetic component for autism. Neurotransmitters, such as serotonin and catecholamines, have been suggested to play a role in the disease since they have an essential function in synaptogenesis and brain development. In this preliminary study, polymorphism of genes implicated in the serotonergic and dopaminergic pathways have been examined in a Spanish population of children diagnosed with autism. Significant association with the disorder was found for the short allele in the promoter of the gene encoding the serotonin transporter (SLC6A4); in addition, a preferential maternal transmission of this allele to affected offspring was observed. (Contains 4 tables.)