The objective of this study was to determine the incidence and prevalence of Type 1 (insulin-dependent) diabetes in Estonia in different sex and age groups. The data collection was based on a centralized retrospective registration of all cases of Type 1 diabetes nationwide using the records of all hospitals and polyclinics where diabetic patients are treated. In 1988, 35 new cases of Type 1 diabetes were diagnosed among children aged 0-14 years (10.3 per 100,000) and 131 among the population over 15 years. The highest incidence of Type 1 diabetes (39.9 per 100,000) was found in the age group 15-19 years. The total number of patients with Type 1 diabetes in Estonia was 2,719 in 1988. The overall prevalence was 1.72 per 1,000. It was highest in the age group 40-49 years (3.04 in males and 2.77 in females). In children under 15 years 204 cases of Type 1 diabetes were identified at the end of 1988. These data suggest that the risk of Type 1 diabetes in Estonia is not low, but is certainly not as high as in Finland where the population is ethnically and linguistically similar and where the highest incidence of Type 1 diabetes is found.