BACKGROUND: In the UK, the use of care planning and written care plans has been proposed to improve the management of long-term conditions, yet there is limited evidence concerning their uptake and benefits. AIM: To explore the implementation of care plans and care planning in the UK and associations with the process and outcome of care. DESIGN AND SETTING: A controlled prospective cohort study among two groups of patients with long-term conditions who were similar in demographic and clinical characteristics, but who were registered with general practices varying in their implementation of care plans and care planning. METHOD: Implementation of care plans and care planning in general practice was assessed using the 2009-2010 GP Patient Survey, and relationships with patient outcomes (self-management and vitality) were examined using multilevel, mixed effects linear regression modelling. RESULTS: The study recruited 38 practices and 2439 patients. Practices in the two groups (high and low users of written documents) were similar in structural and population characteristics. Patients in the two groups of practices were similar in demographics and baseline health. Patients did demonstrate significant differences in reported experiences of care planning, although the differences were modest. Very few patients in the cohort reported a written plan that could be confirmed. Analysis of outcomes suggested that most patients show limited change over time in vitality and self-management. Variation in the use of care plans at the practice level was very limited and not related to patient outcomes over time. CONCLUSION: The use of written care plans in patients with long-term conditions is uncommon and unlikely to explain a substantive amount of variation in the process and outcome of care. More proactive efforts at implementation may be required to provide a rigorous test of the potential of care plans and care planning.