5-Hydroxytryptamine (5-HT), also known as serotonin, is a small molecule synthesized from tryptophan. The serotonergic cells project their axons to a large proportion of neurons, which explains the involvement of 5-HT in many brain physiological mechanisms, behaviors and disease processes. Autism is classified as a pervasive developmental disorder characterized by impairments in three human behavioral domains: a) communication, both verbal and nonverbal, b) socialization (interaction with others) and c) restricted stereotyped behaviors and interests. Symptoms are detectable before the age of three. The etiology of autism is unknown, but it has a genetic basis. Several lines of evidence suggest that 5-HT may be involved in the pathogenesis of autism. For instance, an increase in whole blood and platelet serotonin levels has been reported in about one third of autistic subjects. Furthermore, some autistic patients respond favorably to serotonin re-uptake inhibitors. As serotonergic innervations play an important role in brain development, various neuroanatomical abnormalities associated with autism may be secondary to early dysfunction of this system. Finally, some genetic studies have yielded modest evidence for linkage and association of autism to the serotonin transporter and receptor genes. The possible involvement of genes related to synthesis, release, transport, degradation and signal reception of 5-HT in autism is discussed.