Objectives: To determine the association of weight loss and the onset of dementia of the Alzheimer type (DAT) and to characterize the rate of weight change over time in older adults (aged 65-95 years) who develop DAT vs those who remain without dementia. Design: Rates of weight change were investigated in older adult research participants (N = 449) who were enrolled as control subjects without dementia and followed up longitudinally (6 years on average) at the Alzheimer's Disease Research Center, Washington University School of Medicine. Some individuals (n = 125) eventually developed DAT; the others (n = 324) remained without dementia. Body weight was measured at each annual assessment. Piecewise linear regression and random effects models were used to test longitudinal rates of weight change between the groups. Results: Participants without dementia lost about 0.6 lb per year. For those individuals who developed DAT, about 1 year before the detection of DAT, the rate of weight loss doubled (1.2 lb per year). As a group, participants who eventually developed DAT weighed less (about 8 pounds) at study enrollment (ie, when they did not have dementia) than participants who remained without dementia. Conclusions: Aging with and without DAT is associated with weight loss; however, weight loss may accelerate before the diagnosis of DAT. Specific factors contributing to weight loss are unknown, but these data suggest they operate before the development of DAT. Hence, weight loss may be a preclinical indicator of Alzheimer disease.