We have shown that the number of tryptase-positive mast cells in the duodenal mucosa in psoriasis is increased and that a subgroup of psoriasis patients showed elevated levels of antibodies to gliadin (some of whom also had increased lymphocytes in the duodenal epithelium). Duodenal biopsy specimens from 37 patients with psoriasis (eight mild, 13 moderate and 16 severe) and 22 patients with irritable bowel syndrome (IBS) were examined regarding the presence of tryptase + mast cells. Intraepithelial infiltration by lymphocytes was evaluated and scored from 0 to 3. Patients with psoriasis had 131 +/- 58 mast cells/mm2 (mean +/- SD) and those with IBS 28 +/- 18. Only in four of the 37 psoriasis patients was the number within the range of that in the IBS group. There were no signs of stromal inflammation except in one psoriasis patient. No relationship was found between degree of severity of psoriasis and number of mast cells. In 25 of the 37 specimens from psoriasis patients there was no increase in intraepithelial lymphocytes, whereas seven showed a slight increase (score 1-2) and five a moderate increase (score > or = 2-3). The number of tryptase + mast cells was similar in patients with or without increased intraepithelial lymphocytes. The number of mast cells showed no relation to the presence or absence of antibodies to gliadin. We hypothesize that there are at least two types of abnormalities in the duodenal mucosa in psoriasis, one type that is present in most psoriasis patients and characterized by an increase in mast cells and eosinophils, and another that is present in a subgroup of patients with antibodies to gliadin and an increased number of duodenal intraepithelial lymphocytes. The mechanisms underlying the increase in the number of mast cells and its relevance are not yet known.