Introduction: Chikungunya and dengue are two arboviral diseases transmitted primarily by Aedes albopictus, a mosquito species present in mainland France since 2004. In 2006, they were added to the list of notifiable diseases. Materials and methods: An assessment of the completeness of mandatory reporting in 2014 and 2015 was conducted by cross-referencing the mandatory reporting database with data from a national network of volunteer laboratories performing biological diagnostics for dengue and chikungunya in mainland France. In addition to the completeness analysis, an analysis of reporting delays was conducted to measure the responsiveness of the DO system. Results: The completeness of the DO system for these two arboviral diseases during the study period was estimated at 17% overall. It was significantly higher in Level 1 departments (21%), during the enhanced surveillance season (May to November), and when the diagnosis was confirmed by early tests (PCR or dengue NS1 antigens), with an estimated completeness of 36%. The overall median time between the onset of symptoms and case reporting to the Regional Health Agency (ARS) was 16 days. This time was longer for cases diagnosed by serology (25 days) than by early tests (9.5 days). Conclusion: This report highlights low completeness of case reporting in mainland France, coupled with significant regional disparities. It provides a basis for raising awareness among healthcare professionals (promoting early diagnostic testing, refocusing the chikungunya and dengue surveillance system on reporting confirmed cases). Informing travelers about prevention methods and the importance of seeking medical advice if symptoms develop also remains essential.