Objectives: This paper evaluates the nutritional status in patients with mild-moderate and severe Alzheimer's disease (AD) by bioelectrical impedance vector analysis (BIVA). Design: Cross-sectional study. Setting: Alzheimer Center, SS. Trinità Hospital, Cagliari, and “Monsignor Angioni” Nursing Home, Quartu Sant'Elena (Cagliari, Italy). Participants: 83 free-living patients with mild-moderate Alzheimer's disease (29 men, 54 women), 9 institutionalized women in the severe stage; 468 age-matched controls (202 men, 266 women). Measurements: Mini Nutritional Assessment (MNA), anthropometric (height, weight, BMI), bioelectrical (R, Xc) and biochemical variables (serum albumin) were assessed. Results: Bioelectrical characteristics were significantly different in the patients with mild-moderate AD with respect to controls, indicating low body cell mass (men, T2= 12.8; women, T2=34.9; p<0.01). Women with severe AD showed low body cell mass and dehydration with respect to patients with mild-moderate AD (T2=17.1; p<0.01). The phase angle, R/H and Z/H were significantly correlated (p<0.05) with MNA (phase: r = 0.31; R/H: r =-0.37; Z/H: r =- 0.37) and albumin (phase: r=0.47; R/H: r=-0.36; Z/H: r=-0.36). Conclusion: Alzheimer's disease is characterized by a tendency to malnutrition, present even in the mild-moderate stages, and a tendency to dehydration that appears in the severe stage. The BIVA technique is a promising tool for the screening and monitoring of nutrition and hydration status in Alzheimer's disease.