Background Primary biliary cirrhosis (PBC) is an autoimmune liver disease, and 50% of patients with this disease experience fatigue. This is a debilitating symptom affecting quality of life and resulting in social isolation, which is highlighted by patients as a research priority. PBC is characterised immunologically by the presence of high-titre autoantibodies that are directed at the pyruvate dehydrogenase complex (PDC) and are highly effective at blocking its energy generation function. We hypothesised that if anti-PDC antibodies were a driver of fatigue through bioenergetic dysfunction, then the B-cell-targeting biological agent rituximab (MabThera^®, Roche Products Ltd, Welwyn Garden City, UK) might be a therapeutic option. Objective To assess whether or not rituximab safely improved moderate or severe fatigue in PBC patients. Design A Phase II, double-blind, randomised controlled trial comparing rituximab with placebo in fatigued PBC patients. Randomisation was conducted using a web-based system. Participants received two infusions on days 1 and 15 and were followed up at 3, 6, 9 and 12 months. Setting A single-centre UK study in Newcastle upon Tyne Hospitals NHS Foundation Trust. Participants Seventy-one participants aged ≥ 18 years with PBC and moderate or severe fatigue (score of > 33 on the PBC-40 fatigue domain) were screened. The PBC-40 questionnaire is a fully validated disease-specific health-related quality-of-life measure for use in patients with PBC. Fatigue, with a maximum score of 55, is one of its six domains. Fifty-seven participants were randomised to the trial, 55 of whom reached the primary end-point assessment. Intervention Participants were randomised in a 1 : 1 ratio to receive either rituximab (1000 mg) or a saline intravenous infusion (placebo) on days 1 and 15. The infusions were delivered in a double-blind manner using the same protocol. Main outcome measures The primary outcome measure was the PBC-40 fatigue domain at 3 months, assessed on an intention-to-treat basis. Secondary outcome measures included markers of bioenergetics function (anaerobic threshold and post-exercise muscle pH assessed using magnetic resonance imaging) and physical activity levels. Impact on biochemical markers of liver disease severity was assessed as an experimental outcome. Results Rituximab therapy was safe, with no serious adverse events linked to the drug. There was no statistically significant difference in fatigue score at 3 months between the rituximab and placebo arms [adjusted mean difference –0.9, 95% confidence interval (CI) –4.6 to 3.1]. However, improvement in fatigue was observed in both arms {mean score decreasing from 41.2 [standard deviation (SD) 5.5] to 36.2 (SD 8.4) in the rituximab arm and from 43.0 (SD 5.9) to 38.1 (SD 8.7) in the placebo arm}. There was little difference in any of the secondary outcomes between arms. However, anaerobic threshold improved significantly in the rituximab arm (adjusted mean difference at 3 months 1.41, 95% CI 0.03 to 2.80). No change in muscle bioenergetics characteristics was seen. A suggestive improvement in liver biochemistry was observed. Limitations Recruitment was lower than the original target, leading to a reduction in study power. A clinically significant placebo effect on PBC-40 fatigue scores was seen. Conclusions Rituximab is ineffective for the treatment of fatigue in unselected PBC patients despite metabolic modulation through improvement of anaerobic threshold. Future work Results from the trial demonstrate that metabolic effect of rituximab is not translated into clinical benefit. This will help to guide us to design future trials and when looking at completely different targets. Trial registration Current Controlled Trials ISRCTN03978701, ClinicalTrials.gov identifier NCT02376335 and EudraCT number 2012-000145-12. Funding This project was funded by the National Institute for Health Research (NIHR) Efficacy and Mechanism Evaluation programme and will be published in full in Efficacy and Mechanism Evaluation; Vol. 5, No. 2. See the NIHR Journals Library website for further project information. Additional funding was received from the Medical Research Council and a Department of Health and Social Care subvention.