Objective: To determine the proportion of older adults with Alzheimer's disease presenting to a geriatric clinic with low body mass index (BMI), the proportion of these individuals recognized by clinicians as malnourished, and what patients' characteristics and caregivers' and clinicians' impressions are associated with low BMI. Design: Cross-sectional study. Setting: An outpatient geriatric clinic located in a university-affiliated teaching hospital. Participants: 340 patients with Alzheimer's disease, average age 75 years. Measurements: Individuals with a BMI below 21 were considered at risk of malnutrition. Physical examination and medical information were obtained from patients and caregivers by clinicians using a standardized assessment protocol. Clinicians' impression regarding evidence of malnutrition was obtained. Results: Forty-six patients (16%) had a BMI below 21. Clinicians reported evidence of potential malnutrition in 11 patients, 8 of whom had a BMI below 21. Using logistic regression, we found that women were five times more likely to have a BMI below 21 than men, and that individuals with low cognition were twice as likely to have a BMI below 21 than individuals with higher cognition. Conclusion: The proportion of patients with Alzheimer's disease with a BMI below 21 is similar to that encountered in the general population aged 65+. However, clinicians have difficulty identifying persons at risk of malnutrition according to BMI status. Women with low cognition were at increased risk of having a low BMI. Improvement in the detection of malnutrition is desirable. Further exploration of causal links between cognition and malnutrition is required.