Background: Obesity and the health problems associated with it have substantial economic consequences for health care systems. Little information is available concerning obesity-related problems among people with intellectual disabilities. The aims of this study were to analyse patterns of obesity among children and adolescents with intellectual disabilities in Taiwan and to compare these data with existing national norms in an attempt to identify the scale of the problem of obesity among this population. Methods: Information was collected by postal questionnaire on a total of 279 children and adolescents with intellectual disabilities (age 4-18 years). Results: Sixty-four per cent of participants in the study had multiple disabilities and 40% had additional health needs. In terms of general health status, most of the persons with intellectual disabilities were reported to be in fair or good health, with 9% of them being reported to be unhealthy or very unhealthy. Body mass index (BMI) was related to age, but not to gender, morbidity or prescribed medication at the time of the survey. However, average BMI was higher than relevant population norms reported in Taiwan's Third National Nutrition and Health Survey (1993-1996). The overall prevalence of obesity was 18%, with 14% of participants aged 4-6 years, 22% of participants aged 7-12 years and 16% of participants aged 13-18 years being obese. In a multiple regression analysis of BMI, age was the only significant predictor of obesity, accounting for 8% of variation. Conclusions: The findings suggest that the problem of obesity among children and adolescents with intellectual disabilities should be considered a major public health concern that warrants further attention in health care systems.