With an ageing population and increasing demands on health and social care services, there is growing importance attached to the management of long-term conditions, including maximizing the cost-effectiveness of treatments. In line with this, there is increasing emphasis on the need to keep people both active and participating in daily life. Consequently, it is essential that well developed and validated instruments that can meaningfully assess levels of participation and activity are widely available. Current measures, however, are largely focused on disability and rehabilitation, and there is no measure of activity or participation for generic use that fully meets the standards set by regulatory bodies such as the US Food and Drug Administration. Here we detail a protocol for the development and validation of a new patient-reported outcome measure (PROM) for assessment of participation and activity in people experiencing a variety of health conditions, ie, the Oxford Participation and Activities Questionnaire (Ox-PAQ). The stages incorporated in its development are entirely in line with current regulations and represent best practice in the development of PROMs.