Osteoporosis is a disease caused by compromised bone strength, and individuals with a high peak bone mass at a young age are likely to have a high bone mass in old age. To identify the clinical determinants of peak bone mass in young adult women, 418 southern Chinese women, aged 20-39 years, were studied. Low bone mass was defined as areal bone mineral density (aBMD) Z-score < -1 at either the spine or total hip. Within the cohort, 62 (19.0%) and 86 (26.4%) women had low aBMD at the spine and hip, respectively. Regression model analysis revealed that low body weight (<44 kg) was associated with an 8.3-fold (95% CI, 3.7-18.9) and a 6.8-fold (95% CI, 3.0-15.6) risk of having low aBMD at the spine and hip, respectively. Low body weight was also predictive of low volumetric BMD (vBMD) at the spine (odds ratio (OR) 7.8, 95% CI, 3.1-20.1) and femoral neck (OR 3.0, 95% CI, 1.3-7.1). A body height below 153 cm was associated with a 4.8-fold risk in the small L2-4 bone area (95% CI, 2.3-9.8) and a 3.9-fold risk in the small femoral neck area (95% CI, 1.9-8.1). Delayed puberty (onset of menstruation beyond 14 years) was associated with a 2.2-fold (95% CI, 1.0-4.9) increased risk of having low aBMD at the hip. Physical inactivity was associated with a 2.8-fold risk of low spine vBMD (OR 2.8, 95% CI, 1.1-6.7) and a 3.3-fold risk of low hip aBMD (95% CI, 1.0-10.0). Pregnancy protected against low spine aBMD (OR 0.4, 95% CI, 0.1-1.2) and spine vBMD (OR 0.1, 95% CI, 0.0-1.0), low femoral neck vBMD (OR 0.3, 95% CI, 0.1-1.1) and small L2-4 bone area vBMD (OR 0.3, 95% CI, 0.1-1.1). In conclusion, this study identified a number of modifiable determinants of low peak bone mass in young adult women. Maintaining an ideal body weight, engaging in an active lifestyle, and diagnosing late menarche may enable young women to maximize their peak bone mass and so reduce their risk of osteoporosis in later life.