The Francim network of cancer registries, the Hospices Civils de Lyon, the National Cancer Institute, and Santé publique France have published the results of a new study, funded by the Ligue contre le cancer, on cancer incidence among adolescents and young adults (AJA; ages 15–39). The EPI-AJA study presents (i) the distribution of cancers by age group, (ii) the incidence of “all cancers” from 2000 to 2020, and (iii) trends in the incidence of cancer groups and subgroups over the 2000–2020 period, among the population aged 15 to 39 in the departments of mainland France covered by a general cancer registry. Involving 54,785 individuals, the study was conducted using data from 14 registries covering 19 departments and 24% of the metropolitan population for the distribution of cancers and their incidence over the entire period, and from 11 registries covering 18% of the population for changes in incidence between 2000 and 2020. The study did not aim to extrapolate the results to the national level; the representativeness of the included regions for this population has not been established. While this is the fourth PTP study on cancer incidence among young adults, it is unique in that it focuses on the broader population aged 15–39 (the previous study targeted those aged 15–24) and categorizes cancers according to the new Barr classification published in 2020. Unlike the previous classification, this classification categorizes cancers not only based on the organ they affect but also based on the type of cell involved. Thus, it distinguishes neuroendocrine tumors of the appendix from carcinomas of the appendix. This change in classification limits the ability to compare these new results with previous surveillance data. Changes in cancer incidence from 2000 to 2020 may be linked to (i) the impact of changes in diagnostic and therapeutic practices, (ii) changes in coding and registration by registries, (iii) the decrease or increase in the risk of certain cancers. Over the period, six cancers show a continuous increase in incidence: Hodgkin lymphoma, glioblastoma, liposarcoma, and carcinomas of the kidney, breast, and colorectal tract. Of varying significance across the study area, these increases—with the exception of liposarcoma, which remains very rare among young adults—have already been observed in other countries.