Several studies have shown that infliximab and adalimumab ameliorate fatigue in inflammatory bowel disease. We investigated whether serum levels of these agents above or below a selected threshold influence fatigue severity. In this cross-sectional study, we measured serum concentrations (s-) of infliximab and adalimumab and corresponding anti-drug antibody levels. Therapeutic thresholds were defined as s-infliximab >/= 5.0 mg/L and s-adalimumab >/= 7.0 mg/L. Disease activity was assessed using the Harvey-Bradshaw Index for Crohn's disease, Partial Mayo Score for ulcerative colitis, and C-reactive protein (CRP) and faecal calprotectin levels for both conditions. Fatigue was assessed with the Fatigue Visual Analog Scale and Fatigue Severity Scale, and depression was evaluated with the Hospital Anxiety and Depression Scale, Depression subscale. Of 171 included patients (112 with Crohn's disease, 59 with ulcerative colitis), 66 (38.6%) were on infliximab and 105 (61.4%) were on adalimumab. Scores on the two fatigue scales were similar for serum values above versus below therapeutic thresholds for both drugs and did not differ with versus without anti-drug antibodies against either drug. CRP was numerically higher with infliximab levels below versus above the threshold (p = 0.06), whereas both CRP and faecal calprotectin were increased with adalimumab below versus above the threshold (p = 0.022, p = 0.0242). In patients with inflammatory bowel disease on maintenance therapy, s-infliximab and s-adalimumab levels below or above therapeutic thresholds or the presence of anti-drug antibodies did not affect fatigue severity. Trial Registration: ClinicalTrials.gov identifier: NCT02134054.