Background: To assess the influence of body mass index (BMI) on the progression of dementia. Methods: Sixty elderly outpatients with untreated dementia followed for 12 months. All patients underwent clinical, cognitive, functional, and nutritional assessment at the baseline and after 12 months. Patients were divided into 2 groups by baseline BMI (< or ≥25 kg/m2). Results: Participants with a baseline BMI ≥25 kg/m2 had significantly higher Mini-Mental State Examination (MMSE) scores (21 ± 5.1 vs 15.9 ± 5.5; P < .001), while clinical dementia rating (CDR) and multidimensional prognostic index (MPI) scores were similar in the 2 groups. After 12 months, the MMSE score decreased significantly in both groups compared to the baseline, while the CDR and MPI scores increased significantly for patients with a baseline BMI <25 kg/m2. Conclusion: A BMI cutoff of 25 kg/m2 could be useful for identifying frail patients with dementia who will experience a more rapid global impairment, which could be assessed adequately using multidimensional evaluation tools.