All new cases of Type 1 (insulin-dependent) diabetes mellitus in the 15-29 year age group during the five-year period 1978-1982 were registered using a retrospective technique on a nation-wide basis. A total of 784 newly diagnosed cases were detected, from an average population of 926,192. The degree of ascertainment was almost 90%. The mean yearly incidence for the five-year period was 17.0 per 100,000. The observed incidence is doubled compared to the incidence found in the city of Oslo during the years 1956-1964 (8.8 per 100,000). The male incidence exceeded the female incidence by 12% (p less than 0.05). There was a marked geographic variation in incidence, with a higher incidence in the three southern health regions compared to the two northern, 18.3 vs 13.9 per 100,000 (p less than 0.01). There was a significant seasonal trend in the incidence data (p less than 0.025) with the highest number of new cases detected in the months of January and September and the lowest number in July. In conclusion, the study suggests a two-fold increase of incidence of diabetes mellitus in the age group 15-29 years during the last 2-3 decades and a geographic variation in incidence within the country, pointing to the operation of environmental pathogenic factors.