BACKGROUND AND AIMS: In Crohn's disease, correlation between clinical assessment and disease activity at tissue level is weak. Our aim was to evaluate the value of serum calprotectin as a biomarker for Crohn's disease. METHODS: The STORI trial patients (n=115) were studied at baseline, in clinical remission before infliximab withdrawal, or at the time of relapse after infliximab withdrawal. Forty healthy controls were also studied. Serum calprotectin level was measured by ELISA. Data were analyzed through correlation analyses, Kaplan Meier curves and Cox model, using available Crohn's Disease Activity Index (CDAI), Crohn's Disease Endoscopic Index of Severity (CDEIS), fecal calprotectin and C-reactive protein levels (hsCRP). RESULTS: Median serum calprotectin was 8892 ng/mL (range: 410-125,000 ng/mL) in Crohn disease patients as compared with 1318 ng/mL (range: 215.8-3770 ng/mL) in controls (P<0.0001). Serum calprotectin was significantly higher for active disease (median=19,584 ng/mL) than for inactive disease (median=8353 ng/mL) (P<0.0001). Serum calprotectin correlated with hsCRP (r=0.4092, P<0.0001) and CDAI (r=0.4442, P<0.0001), but not with CDEIS, on the contrary to fecal calprotectin (r=0.6458, 0.5515, 0.2577 with P<0.0001, P<0.0001, P=0.019 respectively). In multivariate analysis, serum calprotectin used as a discrete variable (threshold: 5675 ng/ml), appeared complementary to hsCRP (>5 mg/l) and fecal calprotectin (>250 mug/g) to predict relapse after infliximab withdrawal (P=0.0173, 0.0024 and 0.0002; HR: 3.191, 3.561 and 4.120). CONCLUSIONS: As a CD biomarker, serum calprotectin has a similar profile as hsCRP. It is also complementary to fecal calprotectin and hsCRP for prediction of relapse after infliximab withdrawal.