This chart review study examined the serum vitamin B12 and folate status of 102 geriatric patients newly admitted to a private psychiatric hospital. Only 3.7% were B12 deficient and 1.3% were folate deficient; 4% were anemic. Nevertheless, those with below-median values of both vitamins had significantly lower Mini-Mental State scores than patients higher in one or both vitamins. Patients with "organic psychosis" with a negative family history for psychiatric disorder had significantly lower B12 levels than those with a positive family history. In major depression, folate levels correlated negatively with age at onset of psychiatric illness and length of hospitalization. These data suggest that (1) biochemically interrelated vitamins such as B12 and folate may exert both a separate and a concomitant influence on affect and cognition; (2) poorer vitamin status may contribute to certain geropsychiatric disorders that begin at a later age and lack a familial predisposition.