Since PAP is a stress protein expressed in human pancreas during pancreatitis but also constitutively synthesized in the small intestine, we looked whether its expression would be altered in patients with celiac disease. Serum PAP concentrations were determined consecutively in 54 patients with celiac disease on a free diet (group A), in 47 patients with celiac disease on a gluten-free diet (group B), in 22 patients with other intestinal pathologies but with normal intestinal mucosa (group C), in 14 patients with retarded growth, no gastrointestinal disease and normal intestinal mucosa (group D), and in 17 controls (group E). Serum PAP levels (ng/ml) were significantly higher in group A (127.3 +/- 56.8) than in the other groups (B: 47.2 +/- 20.5; C: 51.5 +/- 32.2; D: 47 +/- 22.8; E: 27.6 +/- 9.0), which were not different from each other. In group A, a positive correlation was observed between serum PAP values and antigluten antibody levels (vs. AGA IgG r = 0.58, p < 0.001; vs. AGA IgA r = 0.66, p < 0.001). Furthermore, 12 patients from group A were evaluated after 10-12 months of gluten-free diet and in all of them PAP serum concentration had decreased (mean +/- SE before the diet 122.5 +/- 36.4, after the diet 48.7 +/- 13.7, p < 0.0001). In addition, we performed an immunocytochemical study to localize PAP in the intestinal mucosa of patients from all groups except E. PAP was localized to the Paneth cells and to some globet cells, in patients with mucosal atrophy as well as in those with normal mucosa with no obvious quantitative difference. We concluded that in patients with celiac disease the active phase of the disease was accompanied by an increased serum concentration of PAP. Further studies are necessary to understand the mechanism leading to PAP elevation in the serum of patients with celiac disease.