Obesity has a negative impact upon mortality and morbidity. Studies report that obesity is more prevalent in individuals with Down syndrome than individuals with intellectual disabilities (ID) not associated with Down syndrome. However, there have been no studies using a methodology of matched comparison groups and findings from previous studies are contradictory. A detailed method was used to identify all adults with ID in Leicestershire. Individuals were invited to participate in a medical examination that included measurement of their height and weight, from which body mass index (BMI) was calculated. For each person with Down syndrome, an individual matched for gender, age and accommodation type was identified, from the Leicestershire ID database. The data for 247 matched pairs is reported. Women with Down syndrome had lower mean height and weight, but greater mean BMI than the matched pairs. Men with Down syndrome had a lower mean height and weight but there was no statistical difference in BMI compared to the matched pairs. Using World Health Organization categories of BMI, women with Down syndrome were more likely to be overweight or obese than their matched pairs (odds ratio 2.17). Men with Down syndrome were more likely to be in the overweight category than their matched pairs but were less likely to be obese (odds ratio 0.85). This study demonstrates that, compared to a matched sample, there is a greater prevalence of obesity amongst women with Down syndrome but not men. As the impact on the health of people with Down syndrome of being overweight or obese is uncertain, this is an area that requires further study.