Background: Obesity prevalence is unusually high among adults with intellectual disability (ID). There is limited and conflicting evidence on obesity prevalence among ambulatory children and adolescents with ID. The present study aimed to estimate obesity prevalence in this group and to compare with population prevalence. Methods: Survey of nine schools (n = 206, 150 boys) for ambulatory children and adolescents with mild-moderate ID in Scotland in 2007. Obesity was defined as measured body mass index (BMI) at or above the 95th percentile relative to UK 1990 reference data, and using the international definition based on BMI. Obesity prevalence observed was compared against Scottish population data on obesity prevalence from the most recent nationally representative survey. Results: Obesity prevalence (at or above 95th percentile for BMI) was 36%, and was significantly higher among those attending secondary schools compared with primary schools (P less than 0.01). Prevalence of obesity was significantly higher than in the general pediatric population in both boys and girls (P less than 0.01). Conclusions: The present study suggests that obesity may be very prevalent among ambulatory children and adolescents with ID, and that increased obesity risk may begin in childhood.