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Aim: To examine seasonal patterns of incidence of Type 1 diabetes mellitus incidence in children aged 0-14 years in Finland, Sweden, Estonia, Latvia and Lithuania during 1983-1992 (1987-1992 for Finland)., Methods: The study used a method that models incidence data using combinations of sine waves to model seasonal variation around a possible linear trend., Results: In Finland, a significant pattern was found for combined sexes and age groups 0-9 and 10-14 years. A significant pattern was also confirmed for 10-14 year-old boys. In Sweden, the best model with significant pattern was found separately for boys and girls and age groups 0-9 and 10-14 years, however, a significant pattern was confirmed for older girls only. A seasonal pattern in older boys in Finland and girls in Sweden was characterized by two cycles with decreased incidence in June and November-December. The pattern among younger children (0-9 or 5-9 years) had one cycle with a decreased incidence in May-June. In Estonia, a significant pattern was found for the age group 0-14 years and combined sexes. No significant seasonal patterns were found in Latvia and Lithuania., Conclusions: The seasonal pattern with two cycles among older children and one cycle only among younger children may indicate different triggers of Type 1 diabetes mellitus for different age groups.