Methods ProtecT is an ongoing randomised trial of localised prostate cancer treatments in men currently aged 53-76 years (ISCRTN 20141297) with six years follow-up. Symptoms and QoL were collected annually by postal questionnaire with a reminder letter for non-responders at six weeks (phase I). Three additional interventions were sequentially investigated to increase retention. Firstly, clinical centre staff commenced telephoning non-responders at four weeks (phase II). A study pen was included with reminder letters and a shortened questionnaire was sent to non-responders at nine weeks (phase III). Questionnaire response rates over six months were compared by phase. 1279 participants were asked in the questionnaires about online completion. 18 participants gave in-depth interviews about trial follow-up which were transcribed verbatim and analysed thematically.