Distributions of age at onset are widely used in the genetic epidemiology of age-dependent diseases. Examples are estimation of recurrent risks in genetic counselling and testing genetic hypotheses in segregation and linkage analyses. In this study, morbidity parameters are defined, including age-specific morbidity rates, morbidity net risk (incidence), and cumulative incidence (population risk, an integrated measure of population susceptibility to the disease at the moment of the study). Age-specific morbidity risks are calculated from the respective morbidity rates, which are analogous to mortality rates used in demography. Population data typically used for calculation of morbidity rates are discussed. Methods of calculation of morbidity rates based on the data of single and interval epidemiological studies are described. Methods for calculating standard errors of these parameters, estimating their statistical reliability, and testing statistical hypotheses are discussed.