Background: Obesity is a major public health concern internationally and this study aimed to measure the prevalence of obesity in adults with intellectual disabilities in comparison with general population data, and examine the factors associated with obesity. Methods: This was a cross-sectional study of all adults with intellectual disabilities, in a defined geographical area, in the context of a primary care health screening programme. A total of 945 adults with intellectual disabilities were involved, all of whom were living in the community. Body mass index (BMI), demographic characteristics, socio-economic deprivation, level of intellectual disabilities and various health parameters were measured. Results: Overall, 39.3% of women and 27.8% of men were obese, compared with 25.1% of women and 22.7% of men in the comparison general population. The mean BMI of women with intellectual disabilities (28.8, range 12.3-59, SD 7.8) was significantly greater than the mean BMI of men with intellectual disabilities (26.7, range 12.6-49, SD 5.9), and women were more likely to be obese than men with intellectual disabilities (chi[superscript 2] = 29.6, P less than 0.001). Regression analyses showed that for both women and men, the risk of overweight and obesity reduced as the severity of intellectual disabilities increased, and Down syndrome was associated with an increased risk of overweight and obesity. Conclusions: There is a need to carry out research to further our understanding of the reasons behind the increased prevalence of obesity in adults with intellectual disabilities. Effective weight management interventions and accessible clinical services are required to reduce the health inequalities experienced by adults with intellectual disabilities.