The relationship between obesity and traumatic dental injuries was evaluated in a study population of 938 6- to 11-year-old schoolchildren from Rome, Italy, and it was suggested by the lifestyle and the physical activity of obese children, which, according to many authors, is different from the lifestyle of other children. The subjects were examined at school: presence and type of dental injury, overjet, incisor protrusion, upper lip incompetence and presence of obesity were recorded. Trauma predisposition was evaluated with a questionnaire investigating the behaviour which may predispose to impacts. Six out of eight questions in this questionnaire concerned the children's lifestyle and the trauma predisposition score was therefore also considered an estimate of lifestyle and physical activity. The dental injury prevalence of the study population was 21.3% and obesity prevalence was 11.4%. Of the obese children 30.8% had dental injuries vs. only 20.0% of non-obese subjects (P = 0.007). Obese children had only enamel and enamel-dentine fractures and the main reported trauma cause was indoor play; conversely, lean children also had other types of injuries and the most frequently reported cause was outdoor play. The main predisposing factors significantly affecting the probability of dental injury, evaluated with multiple logistic regression, were upper lip uncoverage (OR: 1.23; P = 0.02), overjet larger than 3 mm (OR: 1.68; P = 0.001) and obesity (OR: 1.45; P = 0.01). Surprisingly, trauma predisposition was a protective factor (OR: 0.50; P = 0.00001). The results of this study were explained by the significant inverse correlation between obesity and trauma-predisposing behaviour, thus suggesting that subjects frequently playing sports and lively games were not only less obese but also more skillful and, for this reason, less prone to trauma when they fell or sustained impacts.