The present study investigates the effects of 25 or 50 mg tetrahydroaminoacridine (THA), a cholinesterase inhibitor, on auditory mismatch negativity (MMN) event-related response in 19 patients with Alzheimer disease (AD). MMN is produced by the deviant tones representing passive attention and automatic detection of stimulus change. In an inattentive task condition, standard (85%) and deviant (15%) tones were represented in a random order with interstimulus interval of 1 s in separate blocks. THA 25 mg had no effect on MMN in AD patients. In contrast, THA 50 mg diminished MMN in AD subjects. The results suggests that acute treatment of AD subjects with a cholinesterase inhibitor disrupts the passive detection of change in an auditory input.