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Background: Previous phase 2 trials indicated benefit from B-lymphocyte depletion in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS)., Objective: To evaluate the effect of the monoclonal anti-CD20 antibody rituximab versus placebo in patients with ME/CFS., Design: Randomized, placebo-controlled, double-blind, multicenter trial. (ClinicalTrials.gov: NCT02229942)., Setting: 4 university hospitals and 1 general hospital in Norway., Patients: 151 patients aged 18 to 65 years who had ME/CFS according to Canadian consensus criteria and had had the disease for 2 to 15 years., Intervention: Treatment induction with 2 infusions of rituximab, 500 mg/m2 of body surface area, 2 weeks apart, followed by 4 maintenance infusions with a fixed dose of 500 mg at 3, 6, 9, and 12 months (n = 77), or placebo (n = 74)., Measurements: Primary outcomes were overall response rate (fatigue score ≥4.5 for ≥8 consecutive weeks) and repeated measurements of fatigue score over 24 months. Secondary outcomes included repeated measurements of self-reported function over 24 months, components of the Short Form-36 Health Survey and Fatigue Severity Scale over 24 months, and changes from baseline to 18 months in these measures and physical activity level. Between-group differences in outcome measures over time were assessed by general linear models for repeated measures., Results: Overall response rates were 35.1% in the placebo group and 26.0% in the rituximab group (difference, 9.2 percentage points [95% CI, -5.5 to 23.3 percentage points]; P = 0.22). The treatment groups did not differ in fatigue score over 24 months (difference in average score, 0.02 [CI, -0.27 to 0.31]; P = 0.80) or any of the secondary end points. Twenty patients (26.0%) in the rituximab group and 14 (18.9%) in the placebo group had serious adverse events., Limitation: Self-reported primary outcome measures and possible recall bias., Conclusion: B-cell depletion using several infusions of rituximab over 12 months was not associated with clinical improvement in patients with ME/CFS., Primary Funding Source: The Norwegian Research Council, Norwegian Regional Health Trusts, Kavli Trust, MEandYou Foundation, and Norwegian ME Association.