IMPORTANCE: Decision support tools can assist people to apply population-based evidence on benefits and harms to individual health decisions. A key question is whether "personalising" choice within decisions aids leads to better decision quality. OBJECTIVE: To assess the effect of personalising the content of a decision aid for prostate cancer screening using the Prostate Specific Antigen (PSA) test. DESIGN: Randomized controlled trial. SETTING: Australia. PARTICIPANTS: 1,970 men aged 40-69 years were approached to participate in the trial. INTERVENTION: 1,447 men were randomly allocated to either a standard decision aid with a fixed set of five attributes or a personalised decision aid with choice over the inclusion of up to 10 attributes. OUTCOME MEASURES: To determine whether there was a difference between the two groups in terms of: 1) the emergent opinion (generated by the decision aid) to have a PSA test or not; 2) self-rated decision quality after completing the online decision aid; 3) their intention to undergo screening in the next 12 months. We also wanted to determine whether men in the personalised choice group made use of the extra decision attributes. RESULTS: 5% of men in the fixed attribute group scored 'Have a PSA test' as the opinion generated by the aid, as compared to 62% of men in the personalised choice group (χ2 = 569.38, 2df, p< 0001). Those men who used the personalised decision aid had slightly higher decision quality (t = 2.157, df = 1444, p = 0.031). The men in the personalised choice group made extensive use of the additional decision attributes. There was no difference between the two groups in terms of their stated intention to undergo screening in the next 12 months. CONCLUSIONS: Together, these findings suggest that personalised decision support systems could be an important development in shared decision-making and patient-centered care. TRIAL REGISTRATION: Australian New Zealand Clinical Trials Registry (ANZCTR) ACTRN12612000723886.