BACKGROUND:Interleukin (IL)-9 drives gut inflammation, but its role in Crohn's disease (CD) is unclear. We aimed to analyze correlations between serum IL-9 levels and disease severity and to evaluate their predictive value in relation to the clinical efficacy of infliximab (IFX) in patients with CD. METHODS:Between January 2013 and December 2015, 100 consecutive patients with active CD and 50 age- and sex-matched control individuals were recruited from a tertiary center. Their serum IL-9 levels were measured using an enzyme-linked immunosorbent assay. Correlations between the serum IL-9 levels and disease severity were examined. The serum IL-9 level was explored as a predictor of clinical remission and mucosal healing at week 30 in 50 patients for whom IFX therapy was administered. RESULTS:The serum IL-9 levels were significantly higher in the patients with active CD (22.0 pg/mL) than in the control individuals (6.3 pg/mL) (P < 0.001); they differed according to disease severity (moderate-to-severe CD: 29.1 pg/mL versus mild CD: 12.9 pg/mL) (P < 0.001), and they correlated well with the clinical activity of CD. IFX lowered the serum IL-9 level in patients who achieved efficacy at week 30. The areas under the curves for the IL-9 levels at weeks 14 and 30 that could predict clinical remission and mucosal healing at week 30 were 0.803 and 0.752 and 0.746 and 0.781, respectively. CONCLUSIONS:Serum IL-9 levels correlate with disease severity and the clinical efficacy of IFX in patients with CD, and IL-9 may be a promising novel biomarker for CD monitoring.