Empirical studies have recently demonstrated that major and minor depressive disorders occur in 30-50% of stroke patients, and last more than one year without treatment, although they do respond to tricyclic antidepressants. These mood disorders are not strongly associated with severity of impairment, demographic characteristics, social supports or prior personal history, but major depression is often strongly associated with left frontal or left basal ganglia lesions and pre-existing subcortical atrophy. While the aetiology of these mood disorders remains unknown, serotonergic or noradrenergic dysfunction may play a role. Mania is a rare complication of stroke: the clinical presentation and response to treatment are usually the same as mania without brain injury. Post-stroke mania is strongly associated with both a right hemisphere lesion in a limbic-connected area and a second predisposing factor, such as genetic loading for affective disorder, pre-existing subcortical atrophy or seizure disorder. This disorder may be mediated through frontal lobe dysfunction. The lesion method represents a potentially fruitful technique for investigating the mechanisms of affective disorder.