Mood disorders are common, disabling and tend to be recurrent. They carry a high risk of suicide. Maintenance treatment, aimed at the prevention of relapse, is therefore of vital importance. Lithium has been used for some years as the mainstay of maintenance treatment in bipolar affective disorder, and to a lesser extent in unipolar disorder. However, the efficacy and effectiveness of prophylactic lithium therapy has been disputed. Low suicide rates in lithium-treated patients have led to claims that lithium has a specific anti-suicidal effect. If so, this is of considerable importance as treatments for mental disorders in general have not been shown convincingly to be effective in suicide prevention.