OBJECTIVE: To trace new cases of insulin-dependent diabetes mellitus (IDDM) on an ongoing basis and to estimate the incidence rate and prevalence of IDDM in children 0-19 years of age. RESEARCH DESIGN AND METHODS: The estimation of the prevalence ad incidence rate of IDDM was based on, respectively, the prevalence and the rate of new users of insulin. The data were obtained from pharmacy, patient-based, drug dispensing histories in a well-defined population encompassing 257,113 individuals in theNetherlands. RESULTS: The IDDM incidence rate was estimated at 11.5 (CI95 6.6-17.8) per 100,000 person years in 1989-1990 in children 0-19 years of age. The prevalence was estimated at 11.0 (CI95 8.6-13.4) per 10,000 children 0-19 years of age. DISCUSSION: The incidence rate is comparable to that found in a large, national survey among all paediatricians and internists in the Netherlands. The method offers ongoing monitoring of the incidence and prevalence of IDDM in children, 0-19 years of age, againstlow costs. The method is suitable for rapid and cost-efficient recruitment of cohorts of incident IDDM patients, which fosters research to identify and quantify the attribution of different types of aetiologic and prognostic factors of IDDM.