OBJECTIVE To determine insulin-dependent diabetes mellitus (IDDM) incidence rates for the first time in Seoul, Korea, from 1985 to 1988. RESEARCH DESIGN AND METHODS A mail survey was conducted among all hospitals, a total of 136, with > 80 beds in the Seoul area. Of these, (15.4%) hospitals reported patients with newly diagnosed IDDM. RESULTS The average incidence rate of IDDM from 1985 to 1988 was 0.70/100,000 (95% confidence interval [CI]: 0.55–0.89). When stratified by gender, the average incidence rate in boys was 0.58/100,000 (95% CI: 0.42–0.9), and in girls it was 0.78/100,000 (95% CI: 0.56–1.07). The age at onset was similar to that reported in other countries, i.e., more than 50% of the cases were diagnosed between the ages of 10 and 14. The incidence of IDDM increased in parallel with the age-group (0–4 < 5–9 < 10–14 years). Onset seasonality was also similar to that in other reported countries with the most cases occurring during the winter months and the least number of cases during the summer months. CONCLUSIONS The incidence rate of IDDM in Seoul is the lowest reported in the world. This might be effectively considered a baseline incidence rate before exposure to environment or other risk factors. The similar pattern of onset characteristics in Korea to those in other countries suggests that the same etiological factors are operative.