OBJECTIVE: School absenteeism may be an underlying cause of poor school performance among overweight and obese children. We examined the associations between school absenteeism and body mass index (BMI) in a nationally representative sample. DESIGN AND SUBJECTS: We analyzed the data of 1387 children (6--11 years) and 2185 adolescents (12--18 years), who completed an interview and anthropometric measurement as a part of the National Health and Nutrition Examination Survey, 2005Survey, --2008. The CDC 2000 growth chart was used to categorize BMI status, and the number of school days missed during the past 12 months was assessed by asking the proxies or interviewees. RESULTS: The prevalence of obesity and overweight were 18.96 ± 1.44% (s.e.) and 16.41 ± 0.78%, respectively, among study populations. The means of school days missed in the last 12 months were not statistically different between the normal-weight, overweight and obese groups, 3.79±0.56, 3.86±0.38 and 4.31±0.01 days, respectively. However, when 42 days missed per school month was defined as severe absence, the prevalence of severe absence were 1.57%, 2.99% and 4.94% respectively, among 6 --11-year-old children with normal, overweight and obese. The adjusted odds of severe school absence were 2.27 (95% confidence interval ¼ 0.64 --8.03) and 3.93 (1.55 --9.95), respectively, among overweight and obese children compared with normal-weight peers (P for trend test o0.01). No significant association was found among adolescents. CONCLUSION: Increased body weight is independently associated with severe school absenteeism in children but not adolescents. Future research is needed to determine the nature, and academic and social significance of this association.