The purpose of the study was to describe the trend in the incidence of breast cancer in women under 40 in France for the period 1983-2002 and compare it with the trend observed in other age groups. Data from seven cancer registries were analysed. Annual percentage changes were estimated in different age groups using Poisson regression. During the period 1983-2002, breast cancer incidence in women under 40 increased regularly, with an estimated annual percentage change of 0.65 (95% CI: 0.03-1.26). In women aged over 40, the increase is more significant, particularly among women aged 50-74 (2.93% per year). The increase in breast cancer incidence in women under 40 was low and steady over the 20-year period considered. Even though young women did not experience such a major change in diagnostic practices as the development of screening among women aged 50-74, it is difficult to distinguish the effects of possible changes in risk factors and in diagnostic practices in the slight increase observed. (R.A.)