To explore the relationship between birth weight and type 1 diabetes, data from national birth registry and national surveillance of diabetes in Taiwanese schoolchildren were analyzed. From 1992 to 1997, all schoolchildren aged 6-18 years were screened for diabetes by a mass urine survey program in Taiwan Province. This cohort consisted of 1966 children with diabetes and 1780 of randomly selected subjects with normal fasting glycemia. Questionnaires were designed for telephone interviews with students' parents or physicians to classify subjects' types of diabetes. The birth history of each participant was obtained from the Taiwan's Birth Registry. After merging the data, there were 835 subjects, including 277 of type 1 diabetes and 533 of normal fasting glycemia available for the present analyses. The odds ratio (95% CI) for type 1 diabetes, after adjusting age, sex, socioeconomic status, family history of diabetes, birth order, breast-feeding, BMI, and gestational diabetes mellitus was 2.24 (1.11-4.50) for children with low birth weight (<5th percentile, i.e., < or =2600 g) when compared with the referent group of a birth weight of 3000-3542 g (equivalent to the 25-75th percentile). In conclusion, low birth weight was associated with increased risk of type 1 diabetes in Taiwanese schoolchildren.