The aim of the present research is to conduct a fully powered explanatory trial to evaluate the efficacy of a brief self-regulation intervention to increase walking. The intervention will be delivered in primary care by practice nurses (PNs) and Healthcare Assistants (HCAs) to patients for whom increasing physical activity is a particular priority. The intervention has previously demonstrated efficacy with a volunteer population, and subsequently went through an iterative process of refinement in primary care, to maximise acceptability to both providers and recipients. This two arm cluster randomised controlled trial set in UK general practices will compare two strategies for increasing walking, assessed by pedometer, over six months. Patients attending practices randomised to the self-regulation intervention arm will receive an intervention consisting of behaviour change techniques designed to increase walking self-efficacy (confidence in ability to perform the behaviour), and to help people translate their "good" intentions into behaviour change by making plans. Patients attending practices randomised to the information provision arm will receive written materials promoting walking, and a short unstructured discussion about increasing their walking.The trial will recruit 20 PN/HCAs (10 per arm), who will be trained by the research team to deliver the self-regulation intervention or information provision control intervention, to 400 patients registered at their practices (20 patients per PN/HCA). This will provide 85% power to detect a mean difference of five minutes/day walking between the self-regulation intervention group and the information provision control group. Secondary outcomes include health services costs, and intervention effects in sub-groups defined by age, ethnicity, gender, socio-economic status, and clinical condition. A mediation analysis will investigate the extent to which changes in constructs specified by the Theory of Planned Behaviour lead to changes in objectively assessed walking behaviour. This trial addresses the current lack of evidence for interventions that are effective at increasing walking and that can be offered to patients in primary care. The intervention being evaluated has demonstrated efficacy, and has been through an extensive process of adaptation to ensure acceptability to both provider and recipient, thus optimising fidelity of intervention delivery and treatment receipt. It therefore provides a strong test of the hypothesis that a self-regulation intervention can help primary care patients increase their walking. Current Controlled Trials ISRCTN95932902.