In this letter, the author discusses the use of cobalamin (Vitamin B12) to treat patients with organic mood syndrome caused by exposure to chemicals. This syndrome is characterized by chronic depression with cognitive symptoms, temper outbursts & suicidal impulses. Antidepressants & psychotherapy have been found to be ineffective treatments. A role for cobalamin was suggested by research done in 1988. The author presents the case history of a man treated with cobalamin as illustration. (1 ref.)