The aetiology and natural history of Type I (insulindependent) diabetes mellitus are still not known but both genetics and environmental factors contribute to the development of the disease [1±3]. Although HLA genetics have a major role in the aetiology of Type I diabetes, other genes also contribute to the genetic effect, but the mode of inheritance of the disease is not clear [4]. The genetic effect contributes 70±75 % of the susceptibility to Type I diabetes [5,6]. Environmental factors possibly initiate or trigger the process which leads to the destruction of the beta cells and the onset of diabetes [3,7,8].In the late 1970 s epidemiological reports of diabetic children for the first time showed a wide geographical variation in the incidence of Type I diabetes. During the 1960 s to the early 1980 s the data on incidence of Type I diabetes were available for a few populations only, mostly from regions with a high or intermediate risk for this disease. A large number of registries had been established since the mid 1980 s worldwide. The lack of standardized data made it difficult to determine the true magnitude of the worldwide variation in incidence or time trends [9]. The Di- Diabetologia (1999) Abstract Aims/hypothesis. Several reports on the incidence of Type I (insulin-dependent) diabetes mellitus have suggested that the incidence is increasing. The aim of this study was to find out whether the incidence is increasing globally or restricted to a selected populations only and to estimate the magnitude of the change in incidence. Methods. During 1960 to 1996 37 studies in 27 countries were carried out. To fulfil the inclusion criteria the study periods ranged from 8±32 years. The temporal trend was fitted by linear regression, with the logarithm of the age-standardized incidence as the dependent variable and the calendar year as the independent variable. Then, the regression coefficient (´100 %) is approximately the average relative increase in incidence per year (as percentage). Results. Results from the pooled data from all 37 populations showed that the overall increase in incidence was 3.0 % per year (95 % CI 2.6; 3.3, p = 0.0001). The statistically significant increase was found in 24 of 37 populations including all high incidence ( > 14.6 per 100 000 a year) populations. The relative increase was, however, steeper in the populations with a lower incidence. The correlation between logarithm of the incidence and the increase in incidence was r = ±0.56, p = 0.0004. Conclusion/interpretation. The incidence of Type I diabetes is increasing worldwide both in low and high incidence populations. By the year 2010 the incidence will be 50 per 100 000 a year in Finland and also in many other populations it will exceed 30 per 100 000 a year. [Diabetologia (1999

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