The etiology and pathogenesis of autistic spectrum disorders (ASD) are still unknown. Platelet hyperserotonemia has been detected in 25-60% of autistic children. Higher incidence of gastrointestinal problems in people with autism is observed. The aim was compare the expression of platelet 5-HT(2A)r mRNA in autistic and non autistic groups. In a subgroup of patients with gastrointestinal problems an upper gastrointestinal tract endoscopy was performed and additionally the expression of 5-HT(2A) receptor mRNA in the duodenum was assessed. The examination was conducted in 79 children - 51 with ASD and 28 without autistic traits. Statistically significant differences between the study and control groups were proven in gastrointestinal problems. The analyses reveal a significantly higher level of 5-HT(2A)r mRNA in platelets of the study group patients, which could suggest serotonin system dysregulation.