The coexistence of Down's syndrome (DS) and coeliac disease (CD) has been occasionally reported and both diseases are often related to autoimmune disorders. The pathogenetic factor that links CD and DS may be an altered immune system and/or the presence of a common genetic factor. Some epidemiological investigations, performed in patients with CD, showed an increased incidence of DS compared to the natural incidence of this abnormality in the general population. We studied the prevalence of CD in 83 individuals with DS compared to a group of 200 patients with other gastroenterologic disorders and a random scholastic sample of 500 non symptomatic children. IgG and IgA antigliadin antibodies (AGA) were determined in all patients. Antiendomysium antibodies (EmA) were investigated in all the patients of the first group, while in the other two groups, 27 and 108 cases respectively, selected by AGA positivity, were investigated for EmA. The percentage of AGA IgA positivity in the first group was 31.3% (26/83), in gastroenterologic controls 10% (20/200), in scholastic sample 2.8% (14/500), that shows a significant statistical difference. On the contrary EmA were positive in quite a similar percentage in the three groups. Duodenal [correction of Jejunal] biopsies, were performed in 11 DS patients and in 9 of the other two groups. EmA were positive only in the case with subtotal atrophy in all the groups: 5/11 in the first, 2/4 in the second, 2/5 in the third. On the contrary AGA IgA were often positive also in patients with non coeliac histologic findings.(ABSTRACT TRUNCATED AT 250 WORDS)