Several dengue outbreaks occurred in Japan from 1942 to 1945. Dengue fever emerged in Nagasaki in August 1942 and soon spread to other cities such as Sasebo, Hiroshima, Kobe and Osaka, recurring every summer until 1945 and constituting the greatest outbreak in the temperate zone. Domestic outbreaks have not been reported in Japan since then. However, the number of imported dengue cases has increased year by year: 868 imported cases were reported in Japan between 1999 and 2010 according to the Infectious Diseases Control Law. Moreover, 406 imported cases were confirmed to be dengue virus infection among 768 dengue suspected cases received at NIID from 2003 to 2010. A total of 142 cases (35.6%), 103 cases (25.8%) and 62 cases (15.5%) were noted in the 20-29, 30-39 and 40-49 age groups, respectively. Infecting dengue virus serotypes were determined for 280 of the 406 cases. The number of cases infected with each of the 4 serotypes was 98 (35%) with type 1, 78 (28%) with type 3, 72 (26%) with type 2, and 32 (11%) with type 4. Sixty percent of dengue cases were imported from July to October, the summer vacation season in Japan.