Vitamin B12 deficiency is a common occurrence in the elderly and is associated with delirium, dementia, depression, and psychosis. Psychiatric symptoms may occur in the absence of characteristic hematologic or neurologic symptoms suggestive of B12 deficiency. Because psychiatric symptoms can occur in low-to-moderate “normal” vitamin B12 levels, homocysteine or methylmalonic acid levels should also be checked in those with psychiatric symptoms. Importantly, dementia or cognitive decline become irreversible if not treated promptly. Psychosis appears to respond to vitamin B12 replacement, even after prolonged periods of a B12-deficient state, again pointing out the need to check for B12 deficiency in the elderly with psychosis. Future research and follow-up studies are needed.