Objectives With the increasing aging population in Japan, the demand for medical and long-term care is expected to grow. Consequently, it is important to secure sufficient nursing personnel for medical care facilities. Careful monitoring of the allocation of nurses is crucial for providing services that match the needs of the population. However, few studies have examined the distribution of nurses by the type of facility in which they work or identified any associated factors. The objectives of this study are to examine trends in the distribution of nurses working in hospitals and to identify any associated factors such as policy changes, local socioeconomic characteristics, and nurse supply-and-demand-related factors.Methods We conducted an ecological study using public survey data from 2002 to 2011. We focused on 274 secondary medical areas (SMAs) in 38 prefectures from which we could obtain continuous data over the study period. We calculated the number of hospital nurses per 100,000 of the population in each SMA as well as the Gini coefficient. The explained variable was the number of hospital nurses per 100,000 of the population. We employed the following explanatory variables: SMA population, aging population ratio, population density category, per capita income, region, number of nursing school graduates, and nurse wages offered during the previous year. We then examined the association by applying multilevel analysis.Results The number of hospital nurses per 100,000 of the population in the SMAs increased during the period. The Gini coefficient decreased as a general trend but increased in 2007 and 2008. After adjusting for the SMA population and its increase, depending on the year, the number of hospital nurses was positively correlated with higher income per capita, higher aging population ratio, regions other than Kanto, higher number of nursing school graduates, and higher previous-year wages.Conclusion Although the differences in the numbers of hospital nurses across SMAs were lower, and thus improved, the differences tended to expand for 2 years after revision of the medical payment system in 2006. The results show the possibility of the influence of policy changes such as the revision of the medical payment system. The local socioeconomic characteristics, the number of nursing school graduates, and nurse wages were also factors affecting the distribution of hospital nurses.