This study aimed to evaluate the associations among serum 17beta-estradiol (E2), testosterone (T), sex hormone-binding globulin (SHBG), bone geometric properties, and mineral density in 248 healthy girls between the ages of 10 and 13 yr old. The left tibial shaft was measured by peripheral quantitative computed tomography (Stratec XCT-2000; Stratec Medizintechnik, GmbH, Pforzheim, Germany). The cortical bone and marrow cavity areas were expressed as proportions of the total tibial cross-sectional area (CSA). Cortical thickness and total volumetric bone mineral density (vBMD) were also determined. These tibial geometric and densitometric measures were correlated against the serum sex hormone concentrations after controlling for age and body size. The results showed that E2 was negatively associated with marrow cavity proportion (r = -0.19, P = 0.003) and positively associated with cortical proportion and thickness and with total vBMD (r = 0.26, P < 0.001; r = 0.25, P < 0.001; and r = 0.23, P < 0.001, respectively). However, T was not associated with these bone variables. On the other hand, SHBG was positively associated with marrow cavity proportion (r = 0.17, P = 0.007) and negatively associated with cortical proportion and thickness and with total vBMD (r = -0.14, P = 0.029; r = -0.16, P = 0.010; and r = -0.18, P = 0.005, respectively). Total bone CSA did not correlate with E2, T, or SHBG. These results suggest that E2 has a positive effect on bone geometric and densitometric development by suppressing bone turnover at the endocortical surface during the early pubertal period, that SHBG plays an opposite role to E2, and that T has no detectable effect.