This case report describes successful lithium treatment in a patient with long-standing periodic catatonia. Various medical treatments had been tried with meager results. Since 1955, a number of phenothiazines had been tried, also with little improvement. The institution of lithium treatment in 1964 caused a dramatic improvement in the patient's condition. For 9 years the patient remained totally asymptomatic. Lithium treatment was terminated in 1975 because of lithium intoxication with a diabetes insipidus-like syndrome. The patient then became psychotic again and has since experienced periodic phases similar to the previous illness. The diagnostic validity of the case is discussed, and some possible explanations of the favourable response to lithium are mentioned briefly.