Some instruments have been developed to evaluate 'decision effectiveness' such as the 'satisfaction with decision (SWD)' and 'decision conflict (DCS)' scales and are validated for the US context. Patients identify further outcome domains that are not fully covered in these scales. We developed a patient-based outcome measure to evaluate risk communication and decision making effectiveness, addressing these further domains and validated for use in the UK. Formulation of items was based on literature review and key informant interviews. The instrument was piloted and developed (including psychometric assessment) in the context of a trial of decision making and risk communication interventions in general practice. Patients were recruited to the trial with known atrial fibrillation, prostatism, menorrhagia or menopausal symptoms. High response rates (96% in first phase) indicated acceptability to patients. A 20 item patient-based outcome measure, with two sub-scales for 'risk communication' and 'confidence in decision', was produced. It allows for paternalistic, shared or informed choice decision making models. Usage in further studies will facilitate systematic reviews. Consequently, lessons for wider application of these interventions in practice, and policy implications regarding greater patient involvement should then be clearer.