To the editor: In their recent paper "Lithium Salts: 1970-1975" (Ann Intern Med 83:527-533, 1975), Baldessarini and Lipinski briefly mention the leukocytosis (granulocytosis) that occurs in patients taking lithium. The authors hint that the corresponding temporary increase in circulating adrenocorticosteroids is probably not responsible for the persistent leukocytosis. There is evidence suggesting that lithium induces human granulocyte proliferation (1, 2). Circulating (in vivo) unsaturated vitamin B12 binding capacity, an index of total blood granulocyte pool (3), is elevated in patients with granulocytosis who are taking lithium (1). Lithium added to human bone marrow in vitro also increases granulocyte colony formation