WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: Youth with ADHD may be at increased risk for obesity. Medications used to treat ADHD can affect weight. Few studies have investigated possible gender differences in associations between ADHD and obesity. WHAT THIS STUDY ADDS: Nationally representative of US youth aged 8-15 years. Height and weight were measured, and ADHD assessed by structured diagnostic interview and parent report. Associations between ADHD and obesity are reported for males and females to enable gender comparisons. OBJECTIVE: To investigate how associations between attention-deficit/hyperactivity disorder (ADHD) and obesity differ by gender and medication use in a nationally representative sample of US youth in which height and weight were measured. METHODS: Youth age 8-15 (n = 3050) studied in the National Health and Nutrition Examination Survey 2001-2004. Obesity was defined as >/=95th percentile of US body mass index-for-age reference. ADHD was determined by asking parents if child had been diagnosed and using the Diagnostic Interview Schedule for Children IV. Gender-stratified multivariable logistic regression was used to estimate odds of obesity for youth with ADHD (medicated and unmedicated) relative to youth without ADHD. RESULTS: Males with ADHD who were medicated had lower odds of obesity compared to males without ADHD (adjusted odds ratio [OR] = 0.42, 95% confidence interval [CI] = 0.23-0.78). Unmedicated males with ADHD were as likely as males without ADHD to be obese (adjusted OR = 1.02, 95% CI = 0.43-2.42). The odds of obesity for females taking medication for ADHD did not differ statistically from those of females without ADHD (adjusted OR = 1.21, 95% CI = 0.52-2.81). Females with ADHD not taking medication had odds of obesity 1.54 times those of females without ADHD; however, the 95% CI (0.79-2.98) was wide and not statistically significant at alpha = 0.05. CONCLUSIONS: Associations between ADHD and obesity are influenced by treatment of ADHD with medication and may differ by gender. Youth with ADHD who are not treated with medication are as or more likely than youth without ADHD to be obese. the Study of Obesity.