Seasonal patterns in the incidence of insulin-dependent diabetes based on 2,062 cases diagnosed at age 14 years or under in Finland are described for the years 1987-1992. Seasonal patterns were estimated presenting the data as short Fourier series up to three harmonics together with a possible linear trend. This method allows an arbitrary shape for the seasonal effect. Likelihood ratio tests and Akaike's information criterion were used to determine the number of harmonics necessary to model the seasonal pattern and to test differences among age- and sex-specific subgroups in the population. Seasonal patterns in incidence were compared between sexes and between the three 5-year age groups with each controlling for the other's effect. A significant seasonal pattern in the incidence of insulin-dependent diabetes was found for the sexes combined and for two age groups (0-9 and 10-14 years). A statistically significant seasonal pattern could be confirmed for males, but not for females. During a calendar year, one cycle with a decreased incidence of insulin-dependent diabetes in June was found among younger boys. Among older boys, there were two distinct cycles with a decreased incidence, the first in June and the second during November-December. The most visible seasonal pattern was a lower number of cases diagnosed in June, while during the rest of the year the incidence remained relatively stable and high. The average annual incidence was 35.6 per 100,000 persons without any upward peaks.