If the distribution of onset age of disease in a well-defined prevalent (crosssectional) population of patients is known, disease incidence rates specific for age and calendar time period may be estimated, assuming known mortality rates and a dosed population. This paper develops a method of estimation, illustrates this method on Danish diabetes data, and discusses tts general applicability. The prevalent population of diabetic subjects in Fyn County on July 1, 1973 was ascertained from prescriptions, and information on disease onset was obtained from the patients' medical records. In this study only patients with onset of disease before or at age 30 years were studied. The mortality of diabetic subjects in Denmark was estimated from retrospective hospttal data covering the period since 1933, and historical age-specific population sizes of Fyn County were obtained from census data. The incidence of diabetes increases wtth calendar time and with age for most cohorts. The variation with age for a fixed calendar year is more complicated, however, usually displaying a local maximum at about the age of puberty and a higher inddence at the upper end of the studied age range.