Dengue and chikungunya are two arboviral diseases found in the intertropical regions. Transmission occurs from person to person via mosquitoes of the genus Aedes, particularly A. aegypti and A. albopictus. The risk of initiating a transmission chain in mainland France depends, on the one hand, on the risk of the virus being introduced by the arrival of viremic individuals and, on the other hand, on the risk of transmission by competent mosquitoes. A. albopictus has been established since 2005 in the Alpes-Maritimes, since 2006 in Haute-Corse, and since 2007 in the Var and in Southern Corsica. Imported cases have been monitored in mainland France since the start of the chikungunya epidemic in Réunion and since early 2006 for dengue, based on laboratory results from major laboratories (cases with laboratory confirmation obtained from a specimen tested in a mainland laboratory). Between January 1, 2006, and December 31, 2007, 851 imported cases of chikungunya and 658 cases of dengue were identified based on laboratory data. The median age at diagnosis was 50 years [5 months–85 years] for chikungunya cases and 42 years [6 months–84 years] for dengue cases. The male-to-female ratio was 0.86 for chikungunya cases and 1.12 for dengue cases. The trend in the number of chikungunya cases followed the dynamics of the epidemic in the Indian Ocean; the number of dengue cases also followed the development of dengue epidemics, particularly in the French West Indies. In the departments where A. albopictus was established in 2006, 9 imported dengue cases and 8 imported chikungunya cases were reported between May 1 and November 30, the mosquito’s active period. In 2007, within the expanded area of establishment, 22 cases of dengue and 3 cases of chikungunya were reported for the same period. These results provide data for a qualitative assessment of the risk of indigenous transmission of these diseases in southern France. Active entomological and epidemiological surveillance must therefore be continued in order to best guide prevention and control measures and to complete the risk assessment. (R.A.)