Weight loss is a frequent complication of Alzheimer's disease (AD) and a strong predictor of adverse outcomes in patients suffering from this disease. The aim of this study was to determine whether weight loss was a predictor of rapid cognitive decline (RCD) in AD. Four hundred fourteen community-dwelling ambulatory patients with a diagnosis of probable AD and a Mini-Mental State Examination (MMSE) score between 10 and 26 from the REAL.FR (REseau sur la maladie d'ALzheimer FRançais) cohort were studied and followed up during 4 years. Patients were classified in 2 groups according to weight loss defined by a loss of 4% or more during the first year of follow-up. RCD was defined as the loss of 3 points or more in MMSE over 6 months. The incidence of RCD was determined among both groups over the last 3 years of follow-up. MMSE, Katz's Activity of Daily Living scale, Mini-Nutritional Assessment scale, co-morbidities, behavioral and psychological symptoms of dementia, medication, level of education, living arrangement, and caregiver's burden were assessed every 6 months. Eighty-seven patients (21.0%) lost 4% or more of their initial weight during the first year. The incidence of RCD for all patients was 57.6 (95% confidence interval (CI) = 51.6-64.8) per 100 person-year (median follow-up of 15.1 months). In Cox proportional hazards models, after controlling for potential confounders, weight loss was a significant predictor factor of RCD (adjusted hazard ratio (HR) = 1.50, 95% CI = 1.04-2.17). In conclusion, weight loss predicted RCD in this cohort. Whether the prevention of weight loss (by improving nutritional status) impacts cognitive decline remains an open question.