Background: Osteoporosis is a common bone disease, which does not give symptoms until the ultimate outcome, the fragility fracture occurs. Regulation of bone mass is controlled by genetic, environmental and nutritional influences. Peak bone mass, defined as the maximum bone mass accrued, is usually reached by the third decade of life and is an important determinant of future osteoporotic fracture. A number of lifestyle factors, among them physical activity and smoking and even weight at birth are associated with bone mass. However, to what extent these factors are associated to bone mass in young age, peak bone mass, are less clear. Aims: To evaluate peak bone mass and its association with birth weight, recreational levels of physical activity and smoking. To evaluate how bone mass in Swedish young adult women compares with other similarly aged populations and the DXA manufacturer supplied reference values. Methods: 1061 women, aged 25 years at inclusion, were recruited to the PEAK-25 cohort. All participants were measured with DXA. In addition a comprehensive lifestyle questionnaire was completed, including detailed data on physical activities and smoking. Birth anthropometrics were obtained from the Swedish National Board of Health and Welfare. Results: The BMD values of the PEAK-25 cohort were generally higher than equivalently aged European and North American cohorts and the reference cohort incorporated for reference in the DXA scanner. Women with lower birth weight had lower bone mineral content and lower birth weight appears to have a greater negative influence on bone mass than the positive influence of higher birth weight. Recreational levels of physical activity were found to be associated with higher peak bone mass and BMD gains were maximized through regular, high-impact exercise. We found that the quantity of cigarettes consumed, but not smoking duration, is negatively associated with peak bone mass. BMI increases with longer smoking duration and may partly reduce the adverse effects of smoking on bone. Conclusions: If available, ethno-geographically obtained reference data should be used in order to receive more appropriate results from DXA scanning and improve diagnostic accuracy. We have identified risk factors associ-ated with peak bone mass which have the potential for modification. Promoting physical activity, even on recre-ational level, will have beneficial influence on peak bone mass. Further, if not complete cessation of smoking, a reduced number of cigarettes may have beneficial effects on bone health. For bone promoting measures, children with low birth weight ought to obtain additional support.