Patients with ischaemic lesions of the left cerebral hemisphere were examined for depression and intellectual impairment: in non-depressed patients, the severity of impairment was related to both lesion volume and location, as assessed by CT scan analysis. Cognitive impairment in patients with major depression was greater than predicted by lesion volume alone, and when patients were matched for severity of impairment, depressed patients had smaller lesion volumes than the non-depressed. After six months, non-depressed patients had significantly less cognitive impairment than depressed patients who showed no improvement. Both depression and lesion volume were significantly and independently related to cognitive impairment. These findings suggest that post-stroke depression can produce a true dementia in its own right, and that treatment of post-stroke depression might benefit cognitive function.