Background: Persephone is a phase III randomised controlled trial comparing six months of trastuzumab to the standard 12 month duration in patients with HER2 positive early breast cancer in respect of disease free survival, safety and cost-effectiveness. A Persephone sister study, the PHARE trial run by the National Institute for Cancer, successfully closed to recruitment in 2010. A prospective meta-analysis is planned once each trial has reported individually. Methods: A total of 4000 patients will be randomised into each of the two treatment groups. The power calculations assume that the disease-free survival (DFS) of the standard treatment of 12 months trastuzumab will be 80% at 4 years. On this basis, with 5% 1-sided significance and 85% power, a trial randomising 2000 in each arm will have the ability to prove non-inferiority of the experimental arm defining non-inferiority as ‘no worse than 3%’ below the control arm 4 year DFS. Primary outcome is disease-free survival non-inferiority (equivalence) of 6 months trastuzumab compared with 12 months in women with early breast cancer. Secondary outcomes are overall survival non-inferiority (equivalence); expected incremental cost effectiveness; cardiology function and analysis of predictive factors for development of cardiac damage. Two mandatory sub-studies are: Tumour block collection to discover molecular predictors of survival with respect to duration of trastuzumab treatment and blood sample collection, used to discover single nucleotide polymorphisms (SNPs) as genetic/pharmaco-genetic determinants of prognosis, toxicity and treatment outcome. A third optional sub-study is the quality of life questionnaires. Results: Persephone opened to recruitment in October 2007. To date, 1847 patients (46%) of its total have been randomised from 147 UK sites. Recruitment is due to complete by December 2013 and the first planned interim analysis of the primary outcome will be mid-2016. The IDSMC last reviewed the trial in June 2011 and congratulated the Trial Management Group on the conduct of the trial and the quality of the data. No safety concerns were identified, and the IDSMC proposed that the trial continue to planned recruitment.