Osteoporotic fractures are more common in women than men. Although accelerated bone loss following the menopause is recognized as of major importance, it is generally considered that a lower peak adult bone mass in females also contributes to their increased risk of osteoporosis in later life. To examine potential sex differences in peak adult bone mass we studied 29 pairs of dizygotic twins of differing within-pair sex in whom the female twin was premenopausal (mean age 37 years, range 21–55). Bone mineral density (BMD, g/cm2) was measured at the lumbar spine and femoral neck by dual-photon absorptiometry; 22 pairs also had BMD measured in the distal and 21 pairs in the ultradistal radius by single-photon absorptiometry. There was no significant difference in usual dietary calcium intake or tobacco consumption between the twin pairs. Consistent with accepted dogma, BMD at both radial sites were higher (+27%) in the males than their female cotwins. In contrast, there was no sex difference (male versus female) in BMD (mean ± SEM) in the femoral neck (0.96 ± 0.02 versus 0.97 ± 0.03), and surprisingly, the females had a greater lumbar spine BMD than their male cotwins (1.19 ± 0.03 versus 1.26 ± 0.03, p < 0.05). This difference was observed despite the fact that the males were taller (p = 0.033). If the femoral neck BMD values in the females were corrected for this difference in BMI, their values (0.99 ± 0.03 g/cm2) were significantly higher than those in their male cotwin (p < 0.05). Alternatively if the lumbar spine BMD in the female twins was corrected for the sex difference in anterior-posterior vertebral dimension (measured in cadaveric vertebrae) the female values were then 15% greater than those of their male cotwin. Therefore, a sex difference in peak adult bone density at the lumbar spine or femoral neck does not contribute to the higher incidence of osteoporotic fracture at these sites in aging women. Rather, the data suggest that before the menopause the female sex is associated with a bone density greater than expected on the basis of muscle strength or body size or weight.