The Informant Questionnaire on Cognitive Decline in the Elderly individuals (IQCODE) is a reliable, validated informant-based instrument. Most of the studies well support the validity of the IQCODE in dementia screening, but the sensitivity of the rating scale at the early stage during the course of dementia is limited. In this study, we investigate the utility of the IQCODE for patients with mild cognitive impairment (MCI) and the discriminative power of the IQCODE in patients having MCI with and without functional impairment. The samples included mild Alzheimer disease (AD, N = 280), MCI ([N = 657], further divided into 2 subgroups: patients with MCI having functional impairment [MCI-fi, N = 357] and patients having MCI without functional impairment [MCI-fn, N = 300]), and normal cognition (NC, N = 274). The IQCODE, Mini-Mental State Examination (MMSE), and other neuropsychological tests were administered to all participants. Logistic regression and receiver-operating characteristic (ROC) curves were used to evaluate the diagnostic ability of the IQCODE, compared to the MMSE. The optimal cutoff scores of the IQCODE were 3.19 for the MCI (sensitivity/specificity: 0.979/0.714) and MCI-fn (0.900/0.817), 3.25 for the MCI-fi (0.978/0.701), and 3.31 for mild AD (0.893/0.779), while the MMSE was identical, that is 26, for both MCI and its functional normal and functional impaired subgroups (0.892/0.755, 0.867/0.745, and 0.913/0.745, respectively) and 24 for mild AD (0.807/0.836). The discriminating accuracy of the IQCODE was slightly superior to that of the MMSE but did not reach statistical significance. Our study suggests that the IQCODE might be useful in screening for MCI, with hierarchical scores indicating functional normal or impaired.