Aim The principal aim of the present study was to clarify what type of cognitive decline results in a decrease in the ability to carry out activities of daily living (ADL) in patients with Alzheimer's disease (AD). Methods The participants comprised 1384 outpatients with mild AD. We used the Mini‐Mental State Examination (MMSE) and Barthel Index (BI) as indicators of basic ADL, and the Lawton Index (LI) as an indicator of instrumental ADL (IADL). We then analyzed the relationships between MMSE and BI, as well as between MMSE and LI. We also carried out a logistic regression analysis with BI and LI subitems as dependent variables, and MMSE subitems as independent variables. Results For almost all BI and LI subitems, significantly high odds ratios (OR) were noted in MMSE “Copy the design shown” (e.g. dressing OR 3.66, toilet use OR 3.60 and transfers OR 2.80) and “Write a sentence” (e.g. ability to use telephone OR 5.24, laundry OR 2.60, grooming OR 2.50; P < 0.05). Conclusions Visuospatial cognition had an important effect on the decrease in basic ADL and IADL. Furthermore, the subitems with minimal effect on the decrease of basic ADL and IADL differed. Therefore, it appears that specific activities have little effect on the maintenance of ADL and IADL, and that determining residual cognitive function and utilizing this as a means of compensating for decreased ADL is a useful strategy. Geriatr Gerontol Int 2017; ••: ••–••.