Based on clinical features, a group of 31 patients with cluster headache was divided into 1) episodic cluster headache, with months or years of headache free intervals, 2) chronic cluster headache without significant headache free intervals. Chronic type could be subdivided into primary and secondary varieties, secondary being a transformation from episodic cluster headache. A clinical trial of Lithium Carbonate was undertaken. Blood levels of lithium were determined at regular intervals to monitor the therapeutic dosage. Lithium was found to be an effective prophylactic agent in both episodic and chronic cluster headache patients. The percentage of improvement based on headache index was as follows: 55% of patients had more than 90% improvement; 10% of patients showed 60–90% improvement; 15% of patients showed 25–60% improvement; and 20% no improvement. Effectiveness of lithium was evident in less than a week after the initiation of treatment in those who responded. 55% of patients showed mild side effects such as tremor, nausea, diarrhea, abdominal discomfort and lethargy. Only one patient had serious side effects which needed discontinuation of therapy.The beneficial effect of lithium on cluster headaches appear to be independent of its anti-depressant action. Mechanism of action of lithium in cluster headache is not clear.