Background. — Although Alzheimer's disease (AD) is a progressive degenerative condition, there is great intra- and inter-individual variability in rates of cognitive decline. Thus far, little data exist to explain such variability. Studies that have attempted to explain it have often been based on cross-sectional designs, small sample sizes, and clinical population data. They have also failed to correct for level of cognitive function, despite clinical evidence that rate of decline varies among patients with varying levels of cognitive ability. Methods. — This study presents longitudinal data on a community-based sample of 156 patients diagnosed with probable AD, followed annually for one to five years (average age at entry = 79, range 54–91 years). The effect of level of cognitive impairment (as measured by the MMSE and Mattis DRS), demographic characteristics (e.g., education and age), behavioral problems (e.g., agitation), and co-existent health problems (e.g., vascular disease) on rate of decline was investigated via multivariate regression analysis. Results. — Study results indicate that the average rate of decline in cognitive function, as measured by the MMSE and niDRS, becomes more rapid as the disease progresses. Higher education, younger age, and agitation at intake were also significantly related to increased rates of cognitive decline.