This group of patients is described as suffering from manic-depressive illness (manic depression) or bipolar disorder and they represent a relatively small proportion of patients who suffer from depressive illnesses, but are important. This is because the age of onset of these disorders is early, often in the middle twenties as opposed to the average onset for all depression which is in the middle thirties, and therefore the potential for suffering and adverse effects on the patients' lives is magnified.

Manic-depressive (bipolar) illnesses tend to run more in families than most depressions and also to be more severe as well as recurrent. Sometimes they can be dangerous, disruptive of home and business life, and often lead to hospital admission. Sufferers are often treated with drugs, which are generally effective in controlling the symptoms, and which also tend to be recommended as a long-term protection against recurrence.