Prevalence, incidence and duration of a condition or illness in a steady-state population are interrelated in such a way that two of these quantities may be used to obtain the third. Data may be collected in the most expedient manner, either as prevalence or incidence series of cases, and the results expressed as both incidence and prevalence. Information about the distribution of total durations of the condition in an incidence series of cases, or about the distribution of total durations or durations-to-date in a prevalence series is necessary in order to use these relations between prevalence and incidence of a condition or illness. The duration of a condition may be taken as one measure of the effect of the condition or illness on a population, and a "treatment effect" measured by comparing two populations may be termed the etiologic duration. Exact methods are presented for interconverting the distribution of total durations of condition among prevalence and incidence series of cases arising from the same steady-state population. Both prevalence and incidence series of cases may naturally arise in the same epidemiologic study, such as the initiation and conduct of a periodic screening program, and under certain conditions the size and even the direction of the etiologic duration may differ as measured in corresponding prevalence and incidence series of cases.