The primary aim of this research project is to test the effectiveness of a program of detection, diagnosis and treatment of late life depression in primary care. The study consists of a two-armed randomized controlled trial. Detection and diagnosis of depression will take place in consecutive older patients visiting the practices of participating GPs, using the GDS (screening) and primary care version of the CIDI (diagnosis of major depressive disorder). Matched randomization will take place at the practice level. Treatment will consist of standard clinical management, pharmacotherapy and 'supportive contacting'. In the control condition practice will go on as usual. Each arm will include 10 practices. Seven patients will be recruited from each participating practice, resulting in 70 patients per arm. The acute treatment phase will end after 4 months. Follow-ups will be carried out at 8 and 12 months, resulting in a one-year follow-up of all patients. Outcome variables will include the course of depression, well-being, daily functioning and service utilization. A study of cost-effectiveness will be included, as will be studies of effect-modification, process variables and both patient and GP satisfaction. The intervention fits current GP guidelines, which, if proven effective, enhances implementation at a larger scale.