OBJECTIVE: To analyze the change in the age distribution at onset of type 1 diabetes in boys and girls aged 1-14 years during a 32-year period (from 1965 to 1996). RESEARCH DESIGN AND METHODS: Data on the incidence of type 1 diabetes in Finland were obtained from the Central Drug Registry of the Social Insurance Institution for 1965-1986 (6,195 cases) and from the Prospective Childhood Diabetes Registry for 1987-1996 (3,613 cases). The change in age- and sex-specific incidence was estimated by fitting the linear regression with the logarithm of the annual incidence data. Analysis of variance was used to compare the trends between the various age-groups (1-4, 5-9, and 10-14 years) and sexes. RESULTS: The incidence of type 1 diabetes increased predominantly in the younger age-groups. In children aged 1-4 years, the increase was 4.2% per year, and the overall 32-year relative increase was 338%. For children aged 5-9 and 10-14 years, the increase was 2.5 and 1.3% per year, respectively, and the overall relative increase was 116 and 49%, respectively. In boys aged 1-9 years, the increase was greatest from 1965 to 1984, whereas in girls aged 1-9 years, the statistically significant increase occurred between 1985 and 1996. In children aged 10-14 years, the only significant increase was seen in boys from 1965 to 1974 (3.7% per year). CONCLUSIONS: The greatest increase in the incidence of type 1 diabetes mainly occurred in children aged < 5 years. The incidence in young boys has been increasing since the mid-1960s, whereas in young girls, the significant increase began later, around the mid-1970s. In children aged 10-14 years, the increase in incidence has leveled off.

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