The short- and long-term effects of administration of lithium carbonate in cluster headache (CH) have been investigated. Of the 90 patients treated (78 males and 12 females), 68 had episodic CH and 22 had the chronic form of the disease. The doses used were almost always 900 mg/day. Eleven of the 22 patients with chronic CH showed a definite, constant improvement both short and long term. In 7 of the 22 patients, lithium treatment provided excellent results initially but was later followed by some transient worsening; in the remaining 4 only partial benefits were observed initially and treatment proved still less effective after a few months. The effects of cessation of lithium administration after at least five months of continuous treatment were studied in 9 cases. In 6 of them the attacks re-appeared immediately, whereas in 3 the attacks occurred again only after free intervals of four to six months. Of the 68 patients with episodic CH, 26 proved highly responsive to treatment, 26 only partially responsive, and 16 refractory. In 3 cases, after one to three years of continuous treatment, euthyroid goitre developed, which disappeared after the drug was discontinued.