Background: We examined the prevalence of obesity in adults with intellectual disabilities (ID) compared with the general population, and the factors associated with obesity and weight management status, comparing individuals with ID who were overweight or obese to those who were not. Methods: We analysed baseline data (n=1450) from the ongoing 4-year Longitudinal Health and Intellectual Disabilities Study (LHIDS) using a multivariate approach. Measures included body mass index (BMI), demographics, level of ID, diagnoses related to ID, health behaviours (i.e. physical activity, dietary habits, smoking, and alcohol consumption), various health parameters (e.g. mobility limitation, medications), and residential type and location. Results: Compared with the general population, adults (≥18 years) with ID had a higher prevalence of obesity (38.3% vs. 28%) and morbid obesity (7.4% vs. 4.2%). Being female (AOR=1.40, 95% CI=1.09-1.81), having Down syndrome (AOR=2.53, 95% CI=1.86-3.45), taking medications that cause weight gain (AOR=1.80, 95% CI=1.38-2.37), engaging in less moderate physical activity (AOR=0.89, 95% CI=0.79-0.99), and drinking greater amounts of soda (AOR=1.20, 95% CI=1.02-1.42) were associated with higher rates of obesity. Conclusion: Adults with ID, in general, have a high risk of developing obesity, and women with ID have a high risk of developing morbid obesity. Health promotion initiatives should target individuals with the greatest risk. © 2013 MENCAP and International Association of the Scientific Study of Intellectual and Developmental Disabilities and John Wiley & Sons Ltd.