question: Objectives: Anti-TNF agents are effective to treat perianal Crohn's disease (CD). Evidence suggests that Crohn's disease patients with perianal fistulas need higher infliximab (IFX) serum concentrations compared to patients without perianal CD to achieve complete disease control. Our aim was to compare anti-TNF serum concentrations between patients with actively draining and closed perianal fistulas. Methods: A retrospective survey was performed in CD patients with perianal disease treated with IFX or adalimumab (ADL). Fistula closure was defined as absence of active drainage at gentle finger compression and/or fistula healing on magnetic resonance imaging. Results: We identified 66 CD patients with a history of perianal fistulas treated with IFX (n = 47) and ADL (n = 19). Median IFX serum trough concentrations ([interquartile range]) were higher in patients with closed fistulas (n = 32) compared to patients with actively draining fistulas (n = 15): 6.0 µg/ml [5.4-6.9] versus 2.3 µg/ml [1.1-4.0], respectively (p < .001)). A similar difference was seen in patients treated with ADL: median serum concentrations were 7.4 µg/ml [6.5-10.8] in 13 patients with closed fistulas versus 4.8 µg/ml [1.7-6.2] in 6 patients with producing fistulas (p = .003). Serum concentrations of ≥5.0 µg/ml for IFX (area under the curve of 0.92; 95% CI: 0.82-1.00)) and 5.9 µg/ml for ADL (area under the curve of 0.89; 95% CI 0.71-1.00) were associated with fistula closure. Conclusion: Cut-off serum concentrations ≥5.0 µg/ml for IFX and ≥5.9 µg/ml for ADL were associated with perianal fistula closure. Hence, patients with producing perianal fistulas may benefit from anti-TNF dose intensification to achieve fistula closure.