The objective was to test whether the pattern of increase in incidence of type I diabetes in children under 20 y of age varies with age at onset in France during 1988-1997. The French register of insulin-dependent diabetes was based on the direct identification of new cases by a prospective registration in children under the age of 20 y. Data from the French Social Security were used as a secondary independent source of cases. The rate of ascertainment was >/=95% over the 10-y period studied. Data were analyzed using linear regression; departure from a linear trend was tested in each age group. A total of 1,867 children under 20 y of age at the time of diagnosis were included. The incidence rate of type I diabetes in children rose significantly between 1988 and 1997, from 7.41 per 100,000 per year (95% confidence interval: 6.55-8.27) to 9.58 per 100,000 per year (95% confidence interval: 8.64-10.52) with p = 0.0001. The percentage increase was greater in the 0-4 y age group, with a significant departure from linear trend (p = 0.036), reflecting an acceleration of the increase. Incidence rates rose linearly in 5-9 y (average increase 0.43 per 100,000 per year, p = 0.011) and 10-14 y (average increase 0.40 per 100,000 per year, p = 0.002) age groups, whereas it remained stable in the 15-19 y age group (p = 0.77). The incidence rate was significantly higher in boys than in girls (p = 0.005), but the rise in incidence did not differ between the two genders. The rise in incidence of type I diabetes in France was of the same magnitude as observed in the rest of Europe over 10 y. A specific pattern was observed in children under 5 y of age, contrasting with stable rates over 15 y. These data suggest a shift toward a younger age at onset of type I diabetes in childhood, contributing, at least in part, to the observed increase in incidence of type I diabetes in children.