  This paper descibes a new method for deriving incidence rates of a chronic
disease from prevalence data. It is based on a new ordinary differential
equation, which relates the change in the age-specific prevalence to the
agespecific incidence and mortality rates. The method allows the extraction of
longtudinal information from cross-sectional studies. Applicability of the
method is tested in the prevalence of dementia in Germany. The derived
age-specific incidence is in good agreement with published values.
