BACKGROUND: Conflicting reports exist on the correlation of fecal calprotectin (FC), with the endoscopic severity of small bowel lesions of Crohn's disease (CD). This study aimed to analyze the correlation between FC and small bowel lesions observed by small bowel capsule endoscopy (CE). METHODS: This prospective multicenter study involved patients aged 16 to < 60 years with CD of ileal or ileocolonic types without a history of intestinal resection. The participants underwent CE, ileocolonoscopy, and FC within a period of 1 month. Patients with active colonic lesions were excluded. Endoscopic remission (ER) was defined as the absence of an ulcer (>/= 5 mm). The primary endpoint was to determine whether FC could be used to define ER of small bowel lesions in patients with CD. The secondary endpoints were the correlation of CE activity and FC. RESULTS: The study involved 49 patients. The correlation between FC and Lewis score was positive with r = 0.436 (P = .002). The receiver operating characteristic curve constructed for ER and FC had an area under the curve of 0.828 (95% confidence interval, 0.694-0.962; P < .001). The cutoff FC for ER was calculated to be 175 mug/g. The correlation between FC and Capsule Endoscopy Crohn's Disease Activity Index was positive with r = 0.542 (P < .001). CONCLUSIONS: This study revealed a correlation between small bowel CE activity and FC. Under this condition, small bowel ER can be defined as an FC level of < 175 mug/g, and FC was suggested to be a useful biomarker for monitoring small bowel lesions during the application of treat-to-target strategies.