Period prevalence is frequently measured in studies based on administrative data such as that from health maintenance organizations. For example, treated prevalence and drug utilization prevalence are important measures that are typically defined in relationship to a specified time period. Often one wishes to compare administrative data with period prevalences based on national surveys. It may also be of interest to compare period prevalences from two (or more) different data sources. This comparison is not straightforward owing to the problem of "person-time at risk." This article reviews the values and drawbacks of period prevalence as compared with cumulative incidence. L ife table methodology is described for comparing period prevalence data from administrative databases with survey results. This technique can be extended to the comparison of period prevalence observations from two or more administrative data bases. Examples are given pertaining to hypnotic drug use and the treatment of schizophrenia.