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Background and Purpose-Variations in stroke incidence could be explained by changes in vascular and environmental factors that affect the risk of stroke and changes in risk factors that are present in early life. The aim of this study was to identify and measure the effects of 3 interrelated factors, age, calendar period of stroke onset, and birth year cohort, on the incidence rates of stroke from 1985 through 2005. Methods-Age-period-cohort models were used to analyze stroke incidence in Dijon from 1985 to 2005 from a population-based registry that collects data on all stroke patients whatever the type of management, in the public hospital, private hospitals, or at home, in the population of Dijon (150 000 inhabitants). Results-For ischemic stroke, the incidence rose with time in men depending not only on age, but also on the period and cohort effects (Pϭ0.017). For women, the incidence only depended on age (PϽ0.001; incidence rate ratio, 1.085; 95% CI, 1.081 to 1.089). For hemorrhagic stroke, the rise in the incidence with time depended only on age in men, whereas in women, it depended on age, period, and cohort effects (Pϭ0.019). Conclusions-Age, birth cohort, and calendar period contain relevant information to define and explain trends in stroke incidence rates over a long period. (Stroke. 2010;41:2762-2767.)Key Words: age Ⅲ cohort Ⅲ incidence Ⅲ period Ⅲ stroke U nderstanding of the natural history of stroke is improving in Western 1-3 as well as in developing countries. 4 It is therefore possible to identify geographic disparities. 1,5,6 Some studies have already observed a delay in the age at stroke onset: 3 years in both men and women in Sweden, 7 2 years in New Zealand, 8 and 1.5 years only in men in the United Kingdom. 9 However, these results on the delay in the age at stroke onset were obtained comparing the mean age of stroke onset in 2 remote periods without taking into account the variability in trends in the incidence of stroke over time. It therefore seems necessary to take into account the evolution in morbidity over time to explain this possible delay in the age at stroke onset.We used the age-period-cohort model methods proposed by Clayton and Shifflers 10 to describe variations in the stroke incidence rate in Dijon from 1985 to 2005 simultaneously taking into account the effect of age, calendar period of stroke onset, and birth cohort. Materials and Methods Study Area and PopulationThe study population comprised all residents of the town of Dijon in eastern France. According to the national census, the popula- Case AscertainmentThe detailed methodology of case ascertainment has already been described elsewhere. 3,6 Briefly, multiple overlapping sources of information were used to ensure the complete collection of cases: (1) the emergency rooms as well as all the clinical and radiological departments of Dijon University Hospital with a diagnosis of stroke made by 1 neurologist; (2) the emergency rooms and all the clinical departments of the 3 private hospitals of the city and its suburbs with...