Purpose: To show the impact of changing the definition of what is "rural" on the outcomes of a rural medical education program. Methods: A cross-sectional sample of 643 graduates under obligatory rural service and 1,699 graduates after serving their obligation, all from Jichi Medical University (JMU), a binding rural education program in Japan, were used as the data source. Communities were divided into decile groups according to population density, and the cut-off for "rural/nonrural" was altered in order to study its impact on the data. Findings: The rural practice rate of obliged graduates had its peak in the decile groups with the lowest population densities, while the peak rates of postobligation graduates and non-JMU physicians were at the decile groups with the highest population densities. Rural practice rates of all of the 3 groups of physicians increased with the increase in inclusiveness of rural definition. The ratio of rural practice rate of obliged graduates to that of non-JMU physicians ("relative effectiveness") increased remarkably with the increase in exclusiveness of rural definition. The relative effectiveness of postobligation graduates did not substantially increase after the cut-off exceeded a certain point of exclusiveness. Conclusions: Definition of "rural" largely determined the rural practice rate and relative effectiveness of JMU graduates. The results suggest that results of past outcome studies of rural medical education programs are potentially biased depending on how rural is defined.