Neuropsychiatric and behavioral symptoms are frequent and problematic components of Alzheimer's disease (AD). In two previously reported studies, metrifonate was shown to benefit behavioral symptoms as assessed by the Neuropsychiatric Inventory (NPI). In this post hoc analysis, detailed studies were completed to determine the effects of metrifonate on individual symptoms. This study was a retrospective analysis of pooled NPI data from two double-blind, placebo-controlled, multicenter 26-week studies of metrifonate that had achieved similar levels of cholinesterase inhibition. Mild-to-moderate probable AD patients received placebo (n = 222) or metrifonate (n = 450) 30 to 60 mg by weight or a 50-mg fixed dose once daily. At 26 weeks, metrifonate-treated patients had significantly reduced NPI total scores (P = .001) and fewer neuropsychiatric symptoms when compared with placebo-treated patients, including hallucinations (P = .004), agitation/aggression (P = .006), depression/dysphoria (P = .011), apathy (P = .019), and aberrant motor behavior (P = .008). Metrifonate reduced or stabilized neuropsychiatric disturbances in 60% of symptomatic patients. Almost 40% of metrifonate-treated patients had a clinically relevant reduction (> or = 30% decrease in NPI score) in their neuropsychiatric disturbances (P = .002). High proportions of metrifonate-treated patients manifested clinically relevant reductions in anxiety (58%, P = .009), apathy (51%, P = .020), and depression/dysphoria (50%, P = .021) compared to placebo. The metrifonate-associated reductions in NPI scores were evident by week 12 and were maintained for the 26-week study period. There was an overall effect size of metrifonate of approximately 15% on total NPI scores when compared to placebo. Metrifonate significantly reduced many of the psychiatric and behavioral symptoms of AD. The observations suggest that enhancement of cholinergic functions in AD has beneficial effects on behavior.