It is assumed that autistic children suffer from gastrointestinal disorders more often than their peers. Children with ASD are diagnosed with platelet hyperserotoninemia more often than other children. Some gastroenterological disorders manifest themselves in parallel with dysfunctional serotoninergic neurotransmission in the gastrointestinal tract. The aim of this study was to compare the frequency of gastrointestinal problems in autistic children and in the control group and to establish the expression of 5-HT2A serotonin receptor mRNA in autistic children. Based on the performed research the following conclusions were drawn: autistic children report gastrointestinal disorders significantly more often than their peers. The disorders are confirmed through histopathological examination. Gastrological awareness should be maintained in autistic children. The considerably higher level of platelet 5-HT2A receptor mRNA indicates a molecular background of serotonin disorders.