Background: PERSEPHONE is a randomised controlled trial comparing six months of trastuzumab to the standard 12 months in patients with HER2 positive early breast cancer. Methods: 4000 patients (pts) will be randomised into the two arms (1:1). The power calculations assume that the disease-free survival (DFS) of the standard treatment (12 months trastuzumab) is 80% at 4 years. Randomisation of 4000 pts will allow the trial to prove non-inferiority of six months trastuzumab (5% 1-sided significance and 85% power). Non-inferiority is defined as ‘no worse than 3%’ below the control arm (12 month) 4 year DFS. Primary outcome is DFS, and secondary outcomes are overall survival (OS) non-inferiority; cost effectiveness; cardiac function and quality of life. Tumour blocks are collected to research molecular predictors of survival with respect to duration of trastuzumab treatment. Blood samples are analysed for single nucleotide polymorphisms (SNPs) as pharmaco-genetic determinants of prognosis, toxicity and treatment outcome. PHARE, a similar trial from the Institut National du Cancer in France, closed to recruitment in 2010 and presented early data at ESMO 2012. Following this an unplanned interim analysis of PERSEPHONE was presented to the Data Monitoring and Safety Committee (DMSC). PERSEPHONE is funded by the NIHR HTA programme in the UK. Results: PERSEPHONE commenced recruitment in October 2007. At abstract submission, 2593 pts (65%) had been randomised from 144 UK sites. Recruitment is due to complete in September 2014 with the first planned interim analysis of the primary outcome mid-2016. The iDMSC reviewed all data available on HERA and PHARE as well as a PERSEPHONE interim analysis. There were no safety findings or signals that would warrant a change of the study plan and the high quality of data returns was noted. Conclusion: PERSEPHONE continues the active recruitment phase as planned. Preliminary but inconclusive PHARE data have reinforced interest in the PERSEPHONE trial both nationally and internationally. There has been full support from the Breast International Group (BIG) and the international breast cancer community to answer this important shorter duration question. Clinical trial information: 52968807.