This study describes the development of post-stroke depression (PSD) prospectively during the first year post-stroke in 285 unselected stroke patients. An appropriate unselected population-based control group without cerebral pathology is included for comparison. Psychiatric assessment with the Hamilton Depression Rating Scale (HDRS) was undertaken unmodified. PSD was defined as HDRS > or = 13. The one-year incidence of PSD among the 209 survivors able to communicate reliably at 1 month was 41%. Most cases develop within the first months following stroke (79%), the frequency of new cases of PSD at one year being 5%, a level comparable to that in the control group. Depressed and nondepressed stroke patients consistently scored 4 points greater on total HDRS than in the corresponding controls.