BACKGROUND: Instrumental activities of daily living (IADL) impairment can begin in mild cognitive impairment (MCI), and is the core criteria for diagnosing dementia in both Alzheimer's (AD) and Parkinson's (PD) diseases. The Functional Activities Questionnaire (FAQ) has high discriminative power for dementia and MCI in older age populations, but is influenced by demographic factors. It is currently unclear whether the FAQ is suitable for assessing cognitive-associated IADL in non-demented PD patients, as motor disorders may affect ratings. OBJECTIVE: To compare IADL profiles in MCI patients with PD (PD-MCI) and AD (AD-MCI) and to verify the discriminative ability of the FAQ for MCI in patients with (PD-MCI) and without (AD-MCI) additional motor impairment. METHODS: Data of 42 patients each of PD-MCI, AD-MCI, PD cognitively normal (PD-CN), and healthy controls (HC), matched according to age, gender, education, and global cognitive impairment were analyzed. ANCOVA and binary regressions were used to examine the relationship between the FAQ scores and groups. FAQ cut-offs for PD-MCI (versus PD-NC) and AD-MCI (versus HC) were separately identified using receiver operating characteristic analyses. RESULTS: FAQ total score did not differentiate between MCI groups. PD-MCI subjects had greater difficulties with tax records and traveling while AD-MCI individuals were more impaired in managing finances and remembering appointments. Classification accuracy of the FAQ was good for diagnosing AD-MCI (69%, cut-off ≥1) compared to HC, and sufficient for differentiating PD-MCI (38.1%, cut-off ≥3) from PD-CN. CONCLUSION: The FAQ task profiles and classification accuracy differed between MCI related to PD and AD.