Background: The etiology of adult obesity is still poorly understood. It has been shown that over-weight children suffer from adverse psychological events, but less is known about the association of adverse psychological factors among normal weight children and adult obesity. Aim: The aim of this study was to examine if the exposure of perception of being bullied during childhood could be associated with development of adult obesity. Methods: Adult, same-sexed twin pairs discordant for BMI were identified from the Danish Twin Registry. The twins underwent an interview and a physical examination. Data were analyzed by means of a growth-curve model and an intra-pair comparison. This design controls for other influences of early lifestyle and socio economic status and is therefore a powerful tool to study independent effects of specific exposures. Results: In total 236 (81.7%) of the twin individuals identified participated in the study. Participants who reported having been bullied in school, had attained a BMI which was on average 1.4 kg/m² (95% CI = 0.2; 2.5, p = 0.02) higher than those not bullied. Two other questions on specific types of bullying resulted in BMI that were 1.1 kg/m² (CI = 0.1; 2.2, p = 0.03) and 1.9 kg/m² (CI = 0.7; 3.1, p = 0.002) larger than subjects who had not been exposed to bullying. There was a direct association between intra pair differences in BMI and exposure to bullying. Conclusion: The results of the study could indicate that being bullied during childhood seems be associated with adult obesity.