BACKGROUND: Irritable bowel syndrome (IBS) is a common functional gastro-intestinal disorder characterized by discomfort with constipation and/or diarrhea with unclear pathophysiology. We aimed to determine the activities of colorectal eosinophils, neutrophils and epithelial cells by biomarkers in feces reflecting these activities. METHODS: Fecal samples were collected from 185 patients with IBS before and after 8 weeks of placebo or mesalazine treatment and from 40 healthy subjects. Calprotectin, eosinophil derived neurotoxin (EDN), eosinophil cationic protein (ECP), human neutrophil lipocalin (HNL) (pab/765) or dimer, human phospholipase BII-precursor (HPLBII-P) and myeloperoxidase (MPO) were measured by ELISA. Symptom scores were evaluated by diaries. RESULTS: HPLBII-P, HNL (pab/765) and EDN, proteins secreted by intestinal epithelial cells, were elevated in IBS patients as compared to healthy subjects (p < 0.0001-p = 0.008). In contrast, the neutrophil proteins calprotectin, MPO and HNL dimer were unaltered. The eosinophilic protein ECP was lower in IBS (p = 0.001). HNL (pab/765) (p = 0.01) and EDN (p = 0.004) increased in IBS patients after mesalazine treatment. Colo-rectal mucosa showed strong staining of HPLBII-P and western blotting of fecal extracts showed the presence of mainly monomeric, epithelial-associated HNL. CONCLUSIONS: The absence of signs of involvement of neutrophils and eosinophils in IBS suggests that activity of local epithelial cells rather than inflammation may be a major determinant of the disease. The measurements of EDN, HNL (pab/765), and HPLBII-P may serve as potential fecal biomarkers in the study and monitoring of IBS.