The criterion on functional activity in the revised diagnostic criteria for mild cognitive impairment (MCI) seems to be conceptually and operationally problematic.We investigated the predictive validity for dementia of this criterion in 140 patients with MCI who participated in the baseline study of the Korean Longitudinal Study on Health and Aging and completed 18-month follow-up evaluation (mean duration of follow-up, 1.57±0.24 y). Annual conversion rates to dementia were compared between the patients who fulfilled the criterion on functional activity and those who did not. Annual conversion rates to dementia of the patients who fulfilled the criterion on functional activity was 4.76%, whereas that of the patients with MCI who did not was 33.07%, indicating that the criterion on functional activity was a significant negative predictor of dementia (odds ratio=0.12). Re-revision of the criterion on functional activity may be warranted to solve its conceptual and operational ambiguities.