Chronic urticaria (CU) is characterized by recurrent itching skin eruptions caused by mast cell degranulation. Relapses can be provoked by food intake. The aim of this study was to investigate if the mast cell number in the gastroduodenal mucosa is increased in CU patients, and whether mast cell counting by pathologists is clinically useful. We defined two study groups: 50 disease controls (16 Belgians and 34 Italians) and 43 Belgian CU patients. Mast cells were detected using immunohistochemistry for tryptase and CD117. The mast cell number in the disease controls was 20.2/high-power filed (HPF; 133.3/mm(2)) in the stomach and 32.5/HPF (209.2/mm(2)) in the duodenum. There was no difference between Belgian and Italian controls, indicating that dietary habits have no influence on the normal gastroduodenal mast cell number. In CU patients, mast cell numbers were significantly higher: 32.4/HPF (186.0/mm(2)) in the stomach (P<0.0001) and 44.8/HPF (246.0/mm(2)) in the duodenum (P=0.0002). CU is thus associated with mast cell infiltration in the gastroduodenal mucosa, even if patients do not have gastrointestinal symptoms. Mast cell counting in gastroduodenal biopsies of CU patients can be useful in selecting patients who may respond to a therapy with intestinal mast-cell-stabilizing agents.