The present study was conducted to examine interhemispheric electroencephalogram (EEG) coherence at rest and during photic stimulation (5, 10, and 15 Hz) in 10 patients with presenile dementia of the Alzheimer type (AD; mean age at onset, 56 years) and 10 sex- and age-matched control subjects. Compared with the control subjects, the AD patients had significantly lower interhemispheric coherence in the resting EEG for the delta, theta-2, alpha, and beta-1 frequency bands. EEG analysis during photic stimulation also showed that the patients had significantly lower coherence, irrespective of the stimulus frequency. In addition, when we examined the changes in coherence from the resting state to the stimulus condition (i.e., coherence reactivity), significant group differences were found at the brain region primarily involved in visual functioning; the patients had significantly smaller coherence reactivity to photic stimulation at 5 and 15 Hz over the posterior regions. These findings suggest that AD patients have an impairment of interhemispheric functional connectivity in both nonstimulus and stimulus conditions. The findings also suggest a failure of normal stimulation-related brain activation in AD.