1. Evidence is presented to indicate that recurring manic-depressive illnesses are not inevitable.2. "Prophylactic electroshock" prevents some recurrences, but the method requires the prolonged cooperation of the patient and is often distressing and distasteful to the patient. Moreover, electroshock is not without its own hazards.3. "Prophylactic leucotomy" also prevents some recurrences. It does not require the continued cooperation of the patient. Follow-up studies in our series of cases indicate that no patient has had a recurrence of symptoms, the maximum length of postleucotomy observation being approximately 4 years. A longer period of study is, of course, necessary, and we propose to follow these patients through our aftercare department for many years.