IntroductionImmune thrombocytopenia (ITP) is an autoimmune condition that may cause thrombocytopenia-related bleeding. Current first-line ITP treatment is with high-dose corticosteroids but frequent side effects, heterogeneous responses and high relapse rates are significant problems with only 20% remaining in sustained remission with this approach. Mycophenolate mofetil (MMF) is often used as the next treatment with efficacy in 50%–80% of patients and good tolerability but can take up to 2 months to work.ObjectiveTo test the hypothesis that MMF combined with corticosteroid is a more effective first-line treatment for immune thrombocytopenia (ITP) than current standard of corticosteroid alone.Methods and analysisDesignMulticentre, UK-based, open-label, randomised controlled trial.SettingHaematology departments in secondary care.ParticipantsWe plan to recruit 120 patients >16 years old with a diagnosis of ITP and a platelet count <30x109/L who require first-line treatment. Patients will be followed up for a minimum of 12 months following randomisation.Primary outcomeTime from randomisation to treatment failure defined as platelets <30x109/L and a need for second-line treatment.Secondary outcomesSide effects, bleeding events, remission rates, time to relapse, time to next therapy, cumulative corticosteroid dose, rescue therapy, splenectomy, socioeconomic costs, patient-reported outcomes (quality of life, fatigue, impact of bleeding, care costs).AnalysisThe sample size of 120 achieves a 91.5% power to detect a doubling of the median time to treatment failure from 5 to 10 months. This will be expressed as an HR with 95% CI, median time to event if more than 50% have had an event and illustrated with Kaplan-Meier curves. Cost-effectiveness will be based on the first 12 months from diagnosis.Ethics and disseminationEthical approval from NRES Committee South West (IRAS number 225959). EudraCT Number: 2017-001171-23. Results will be submitted for publication in peer-reviewed journals.Trial registration number NCT03156452