Background: To explore the prevalence of obesity and related secondary conditions associated with obesity in adolescents with intellectual/developmental disabilities (IDD). Methods: In total, 461 parents of adolescents with IDD (M = 14.9 year, SD = 1.9) across 49 US states completed a web-based survey containing questions related to their child's health status, including body weight and existing health conditions. Results were compared with published data for youth without disabilities. Results: Adolescents with autism and Down syndrome were two to three times more likely to be obese than adolescents in the general population. Secondary health conditions were higher in obese adolescents with IDD compared with healthy weight adolescents with IDD including high blood pressure, high blood cholesterol, diabetes, depression, fatigue, liver or gallbladder problems, low self-esteem, preoccupation with weight, early maturation and pressure sores. Conclusion: Obesity is as much of a health problem in youth with IDD as it is among youth without disabilities and, in certain disability groups, is a significantly greater health problem. Obese youth with IDD have a high number of obesity-related secondary conditions predisposing them to greater health problems as they transition into adulthood. Federal and local initiatives to reduce obesity among youth in the general population must recognise the need for interventions that are also relevant (i.e. accessible and effective) for youth with IDD. © 2010 The Authors. Journal of Intellectual Disability Research © 2010 Blackwell Publishing Ltd.