The Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE) is widely used as a complementary screening tool for dementia. However, there are few studies concerning the efficacy of the IQCODE for assessing the severity of cognitive impairments in patients with Alzheimer's disease (AD). We aimed to evaluate the efficacy of the IQCODE for assessing the severity of dementia in patients with AD. According to the clinical dementia rating (CDR), 394 patients with AD were enrolled and classified into three groups: mild, moderate and severe groups. The IQCODE scores of each group were determined by interviewing the informants with the short version of the 16-item IQCODE. The correlations of the IQCODE score with the Mini-Mental State Examination (MMSE), the Mattis Dementia Rating Scale (DRS) and the Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-Cog) were analysed. Statistical analyses were conducted to examine the differences in the IQCODE scores among the three groups. The validity coefficients of the IQCODE with the MMSE, DRS and ADAS-Cog were - 0.528, - 0.436, and 0.477, respectively. The sensitivity was 66.1%, and the specificity was 59.8% when using a cut-off score of 65 to discriminate between mild-moderate dementia. When 75 was used as the threshold between moderate-severe dementia, the sensitivity and the specificity were 73.9 and 67.7%, respectively. The IQCODE is moderately effective for assessing the severity of cognitive impairment in patients with AD.