BACKGROUND: Late-life depression is associated with chronic illness, disability, and a poor prognosis. Primary care management may be in need of improvement. AIM: To compare the effects of an intervention programme that aims to improve the identification, diagnosis, and treatment of depression in patients aged > or =55 years with the effects of usual care. DESIGN OF STUDY: Cluster randomised controlled trial. SETTING: General practices in the Netherlands. METHOD: Trained GPs performed the intervention and their practice assistants conducted the screenings. Patients were screened with the 15-item Geriatric Depression Scale (GDS-15) and given a consultation with the GP who diagnosed depression with the mood module of the Primary Care Evaluation of Mental Disorders (PRIME-MD). Antidepressant treatment was proposed. Primary outcomes were measured with the Montgomery Asberg Depression Rating Scale (MADRS). Trained independent research assistants performed independent evaluations in both arms. RESULTS: Eighteen practices (23 GPs) were allocated to the intervention and 16 practices (20 GPs) to usual care. From June 2000 to September 2002, 3937 patients were screened; 579 patients had a positive score on the GDS-15, 178 had major depression, of whom 145 participated in the trial. MADRS scores for the intervention group dropped from 21.66 at baseline to 9.23 at 6 months, and the usual care group from 20.94 at baseline to 11.45 at 6 months. MADRS scores decreased during the year in both arms. For the intervention group, these scores increased between 6 and 12 months. CONCLUSION: The programme resulted in lower MADRS scores in the intervention group than in the usual care group, but only at the end of the intervention, at 6 months after baseline.