Additional Study Information: This NIH funded research study involves a 12-week double blind treatment trial in which Alzheimer's disease (ages 55-95) who have agitation/aggression with or without psychosis (hallucinations or delusions) will receive either low-dose lithium or placebo. Patients are allowed to receive other specific psychiatric medications if needed during the study, and will be closely monitored by physicians who specialize in memory disorders. Patients will receive open treatment with lithium, if clinically indicated, after completing the 12-week double blind trial.