[en] BACKGROUND & AIMS: Infliximab is an antibody against tumor necrosis factor-alpha that is used to treat patients with moderate to severe Crohn's disease (CD). C-reactive protein (CRP) is a marker used to identify and follow individuals with CD. We analyzed changes in levels of CRP in a large cohort of patients with CD undergoing treatment with infliximab. METHODS: Serial levels of CRP were analyzed in 718 CD patients. Blood was collected before each infusion; a total of 8845 CRP levels were available for analysis. The correlations between CRP levels and need for dose adjustment, outcomes, and mucosal healing (based on endoscopic analysis of 253 patients) were evaluated. Therapy adjustment was considered successful if therapy continued without need for change. Subgroup analysis was performed by using data from 268 patients who received 8 weeks of maintenance therapy. RESULTS: More patients with high baseline levels of CRP responded to infliximab than patients with normal levels (90.8% vs 82.6%; P = .014). Early normalization of CRP levels correlated with sustained long-term response (P < .001). CRP levels remained significantly higher among patients who lost their response to infliximab, compared with those with a sustained response (P = .001). At time of loss of response, CRP levels were significantly increased (median, 11.2 mg/L) and did not return to baseline levels (median, 18.2 mg/L; P = .039). CRP correlated with mucosal healing (P = .033). CONCLUSIONS: CRP is a good marker of disease activity in patients treated with infliximab. Increased levels of CRP indicate mucosal inflammation and a likelihood of clinical relapse.