BACKGROUND & AIMS: We performed a multicenter study to determine whether transabdominal bowel wall ultrasonography, a noninvasive procedure that does not require radiation, can be used to monitor progression of Crohn's disease (CD). METHODS: We performed a 12-month prospective, noninterventional study at 47 sites in Germany, from December 2010 through September 2014. Our study included 234 adult patients with CD who experienced a flare, defined as Harvey-Bradshaw index score of >= 7. All patients received treatment intensification, most with tumor necrosis factor antagonists. Ultrasound parameters and clinical data were assessed at baseline and then after 3, 6, and 12 months. The primary endpoint was the change in ultrasound parameters within 12 months of study enrollment. RESULTS: All patients included had bowel wall alterations either within the terminal ileum and/or segments of the colon. After 3 and 12 months, ultrasonographic examination showed significant improvements of nearly all ultrasound parameters, including reductions in bowel wall thickening or stratification, decreased fibrofatty proliferation, and increased signals in color Doppler ultrasound (P < .01 for all parameters at months 3 and 12). Median Harvey-Bradshaw index scores decreased from 10 at baseline to 2 after 12 months. Improvement in bowel wall thickness correlated with reduced levels of C-reactive protein after 3 months (P <= .001). CONCLUSIONS: In a multicenter prospective study, we found that ultrasonographic examination can be used to monitor disease activity in patients with active CD. Bowel ultrasonography seems to be an ideal follow-up method to evaluate early transmural changes in disease activity, in response to medical treatment. German