Our aim was to determine the incidence of Type 1 diabetes mellitus (T1DM) in the 15-39-yr-old Lithuanian population from 1991 to 1997. A specifically developed communications system with all endocrinologists/diabetologists and general practitioners involved in diabetes care covering 100% of the sample Lithuanian population was the initial data source. Annual reports from regional endocrinologists/diabetologists, death certificates and patients' lists from Diabetes Societies remained as secondary sources for case ascertainment. The annual incidence was calculated per 100,000 persons/yr. The trend in incidence was estimated by fitting the linear regression model with the annual incidence data. The total of 769 new cases (496 males and 273 females) of T1DM were recorded among the 15-39-yr-old population during the period 1 January 1991-31 December 1997. The average age-standardised incidence rate was 7.78 per 100,000 persons/yr [95% Poisson distribution confidence interval (CI) 7.25-8.35] and was slightly higher among males (9.68 per 100,000 persons/yr, 95% CI 8.87-10.57) than among females (5.71 per 100,000, 95% CI 5.07-6.43). Male/female ratio was 1.82 (95% CI 1.69-1.95). Results of the linear regression model showed that the incidence of T1DM in the 15-39-yr-old population had a tendency to increase. The incidence data of T1DM in Lithuania are appropriate to those in Poland and lower than in other countries of the Baltic Sea region.