BACKGROUND: Fatigue is a common problem in immune-mediated inflammatory disease (IMID) patients, significantly impacting their quality of life. OBJECTIVES: In this study, we describe the pattern and characteristics of fatigue as a patient-reported adverse drug reaction (ADR) of biologics, and compared patient and treatment characteristics with patients reporting other ADRs or no ADRs. METHODS: In this cohort event monitoring study, the description and characteristics of fatigue reported as a possible ADR in the Dutch Biologic Monitor were assessed and analysed for commonly recurring themes or patterns. Baseline and treatment characteristics of patients with fatigue and patients reporting other ADRs or no ADRs were compared. RESULTS: Of 1382 participating patients, 108 patients (8%) reported fatigue as an ADR of a biologic. Almost half of these patients (50 patients, 46%) described episodes of fatigue during or shortly after biologic injection, which often recurred following subsequent injections. Patients with fatigue were significantly younger than patients with other ADRs or patients without ADRs (median age for patients with fatigue, 52 years; median age for patients with other ADRs, 56 years; and median age for patients without ADRs, 58 years); significantly more often smoked (25% vs. 16% and 15%); used infliximab (22% vs. 9% and 13%), rituximab (9% vs. 3% and 1%) or vedolizumab (6% vs. 2% and 1%); and significantly more often had Crohn's disease (28% vs. 13% and 13%) and other comorbidities (31% vs. 20% and 15%). Patients with fatigue significantly less frequently used etanercept (12% vs. 29% and 34%) or had rheumatoid arthritis (30% vs. 45% and 43%). CONCLUSIONS: IMID patients may experience fatigue as a postdosing effect of biologics.