OBJECTIVE: The overall aims of the ADDITION study are to evaluate whether screening for prevalent undiagnosed Type 2 diabetes is feasible, and whether subsequent optimised intensive treatment of diabetes, and associated risk factors, is feasible and beneficial. DESIGN: Population-based screening in three European countries followed by an open, randomised controlled trial, SUBJECTS AND METHODS: People aged 40-69 y in the community, without known diabetes, will be offered a random capillary blood glucose screening test by their primary care physicians, followed, if equal to or greater than 5.5 mmol/l, by fasting and 2-h post-glucose-challenge blood glucose measurements. Three thousand newly diagnosed patients will subsequently receive conventional treatment (according to current national guidelines) or intensive multifactorial treatment (lifestyle advice, prescription of aspirin and ACE-inhibitors, in addition to protocol-driven tight control of blood glucose, blood pressure and cholesterol). Patients allocated to intensive treatment will be further randomised to centre-specific interventions to motivate adherence to lifestyle changes and medication. Duration of follow-up is planned for 5 y. Endpoints will include mortality, macrovascular and microvascular complications, patient health status and satisfaction, process-of-care indicators and costs.