Projections of the incidence and prevalence of disease are important for public health planning. This paper describes methods for projecting the incidence and prevalence of a chronic disease in ageing populations. The approach uses age-specific disease incidence rates together with assumptions about survival to reconstruct disease prevalence. The methods can be used to evaluate the potential impact of public health interventions that may prevent disease or prolong survival. We used the methods to project the future prevalence of Alzheimer's disease in the United States. We found that the prevalence of Alzheimer's disease will nearly quadruple over the next 50 years. Although projections of the absolute prevalence are sensitive to assumptions about the age-specific incidence rates of disease, the proportionate growth is relatively insensitive. The increase in prevalence results from the ageing of the U.S. population. In order to perform the calculations, we have assembled U.S. Census population projections and U.S. mortality rates into computer software that is available from the authors at www.jhsph.edu/Departments/Biostats/software.html.