The peak bone mass achieved at skeletal maturity is considered to be an important contributor to bone strength during later life. Understanding the role of environmental factors on peak bone mass development could contribute to public health strategies for the prevention of osteoporosis. In the Western Australian Pregnancy Cohort (Raine) Study that started with 2,900 pregnant women in 1989, we examined the associations of maternal vitamin D status, hypothalamus-pituitary-adrenal (HPA) axis function, dietary pattern and sedentary behaviour with total body bone mass measured at 20 year in the offspring. We found that maternal vitamin D deficiency at 18 weeks gestation (serum 25OHD<50 nmol/L) was associated with 2.7% lower total body size-adjusted bone mineral content (BMC) in the offspring at 20 years (1). For hormonal factors, plasma and salivary cortisol measured at the baseline of a stress test at 18 years were negatively associated with size-adjusted BMC at 20 years in males but not in females (2). For lifestyle factors, a dietary pattern characterized by high intakes of protein, calcium and potassium with high factor loadings for low-fat dairy products, whole grains and vegetables at 14 but not 17 years was positively associated with size-adjusted BMC at age 20 (21.9g increase per SD increase in Z-score) (3). From data collected concerning TV watching from 5 to 20 years of age, sedentary behaviour represented by consistently high TV watching was associated 7.3% and 3.9% lower total body BMC in males and females, respectively, compared with those with low TV watching (≥14 vs <14 hrs/week). These findings emphasize the significance of several environmental factors during development years for the achievement of optimal peak bone mass, and the importance of considering gender and development stages when evaluating these associations.