We investigated risk factors for fracture among young adults, particularly body mass index (BMI) and physical activity, which although associated with fracture in older populations have rarely been investigated in younger people. Methods: In 2009, 4 years after initial recruitment, 58 204 Thais aged 19 to 49 years were asked to self-report fractures incident in the preceding 4 years. Conditional logistic regression was used to calculate odds ratios (ORs) and 95 CIs for associations of fracture incidence with baseline BMI and physical activity. Results: Very obese women had a 70 increase in fracture risk (OR = 1.73, 95 CI 1.21-2.46) as compared with women with a normal BMI. Fracture risk increased by 15 with every 5-kg/m2increase in BMI. The effects were strongest for fractures of the lower limbs. Frequent purposeful physical activity was also associated with increased fracture risk among women (OR = 1.52, 95 CI 1.12-2.06 for 15 episodes/week vs none). Neither BMI nor physical activity was associated with fracture among men, although fracture risk decreased by 4 with every additional 2 hours of average sitting time per day (OR = 0.96, 95 CI 0.93-0.99). Conclusions: The increase in obesity prevalence will likely increase fracture burden among young women but not young men. While active lifestyles have health benefits, our results highlight the importance of promoting injury prevention practices in conjunction with physical activity recommendations, particularly among women.