BACKGROUND & AIMS: There is a need for a scoring system that provides a comprehensive assessment of structural bowel damage, including stricturing lesions, penetrating lesions, and surgical resection, for measuring disease progression. We developed the Lemann Index and assessed its ability to measure cumulative structural bowel damage in patients with Crohn's disease (CD). METHODS: We performed a prospective, multicenter, international, cross-sectional study of patients with CD evaluated at 24 centers in 15 countries. Inclusions were stratified based on CD location and duration. All patients underwent clinical examination and abdominal magnetic resonance imaging analyses. Upper endoscopy, colonoscopy, and pelvic magnetic resonance imaging analyses were performed according to suspected disease locations. The digestive tract was divided into 4 organs and subsequently into segments. For each segment, investigators collected information on previous operations, predefined strictures, and/or penetrating lesions of maximal severity (grades 1-3), and then provided damage evaluations ranging from 0.0 (no lesion) to 10.0 (complete resection). Overall level of organ damage was calculated from the average of segmental damage. Investigators provided a global damage evaluation (from 0.0 to 10.0) using calculated organ damage evaluations. Predicted organ indexes and Lemann Index were constructed using a multiple linear mixed model, showing the best fit with investigator organ and global damage evaluations, respectively. An internal cross-validation was performed using bootstrap methods. RESULTS: Data from 138 patients (24, 115, 92, and 59 with upper tract, small bowel, colon/rectum, and anus CD location, respectively) were analyzed. According to validation, the unbiased correlation coefficients between predicted indexes and investigator damage evaluations were 0.85, 0.98, 0.90, 0.82 for upper tract, small bowel, colon/rectum, anus, respectively, and 0.84 overall. CONCLUSIONS: In a cross-sectional study, we assessed the ability of the Lemann Index to measure cumulative structural bowel damage in patients with CD. Provided further successful validation and good sensitivity to change, the index should be used to evaluate progression of CD and efficacy of treatment. ; Peer reviewed