Background: There are great geographical differences in the incidence of type I diabetes mellitus. Aim: To determine the incidence rate of type 1 diabetes mellitus (DM1) in the Metropolitan Region of Santiago, Chile from January 1, 2000 to December 31, 2004 and to observe the distribution of cases in the different counties of Santiago. Material and methods: All the cases diagnosed with DM1 in the Metropolitan Region who fulfilled the following requirements were included in the study: age of onset <15 years, insulin treatment from onset, permanent residency in the area, and a diagnosis made between January, 2000 and December, 2004. Results: The incidence of DM1 was 6.58/ 100.000 inhabitants/year, and showed a significant increase from 2001 to 2004 (5.44 and 8.33 inhabitants/year, respectively, p <0.04). The incidence of DM1 also increased significantly in children younger than 4 years old. The incidence by counties exhibited large differences, ranging from 1,5 to 26,6/100.000 inhabitants. Counties with higher income, urbanization and low aborigine component showed a high incidence rate of type 1 diabetes. Conclusions: In the Metropolitan Region of Santiago, an increase of the incidence of DM1 has occurred in the period 2000-2004, especially in children younger than 4 years old. Large differences among counties were observed.