The human brain can anatomically combine task-relevant information from different sensory pathways to form a unified perception; this process is called multisensory integration. The aim of the present study was to test whether the multisensory integration abilities of patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD) differed from those of normal aged controls (NC). A total of 64 subjects were divided into three groups: NC individuals (n = 24), MCI patients (n = 19), and probable AD patients (n = 21). All of the subjects were asked to perform three separate audiovisual integration tasks and were instructed to press the response key associated with the auditory, visual, or audiovisual stimuli in the three tasks. The accuracy and response time (RT) of each task were measured, and the RTs were analyzed using cumulative distribution functions to observe the audiovisual integration. Our results suggest that the mean RT of patients with AD was significantly longer than those of patients with MCI and NC individuals. Interestingly, we found that patients with both MCI and AD exhibited adequate audiovisual integration, and a greater peak (time bin with the highest percentage of benefit) and broader temporal window (time duration of benefit) of multisensory enhancement were observed. However, the onset time and peak benefit of audiovisual integration in MCI and AD patients occurred significantly later than did those of the NC. This finding indicates that the cognitive functional deficits of patients with MCI and AD contribute to the differences in performance enhancements of audiovisual integration compared with NC.