BACKGROUND: Shorter estrogen exposure from puberty onset to peak bone mass attainment may explain how late menarche is a risk factor for osteoporosis. The influence of menarcheal age (MENA) on peak bone mass, cortical, and trabecular microstructure was studied in 124 healthy women aged 20.4 +/- 0.6 (sd) yr. METHODS: At distal radius, areal bone mineral density (aBMD) was measured by dual-energy x-ray absorptiometry, and volumetric bone mineral density (BMD) and microstructure were measured by high-resolution peripheral computerized tomography, including: total, cortical, and trabecular volumetric BMD and fraction; trabecular number, thickness, and spacing; cortical thickness (CTh); and cross-sectional area (CSA). RESULTS: Median MENA was 12.9 yr. Mean aBMD T score of the whole cohort was slightly positive. aBMD was inversely correlated to MENA for total radius (R = -0.21; P = 0.018), diaphysis (R = -0.18; P = 0.043), and metaphysis (R = -0.19; P = 0.031). Subjects with MENA more than the median [LATER: 14.0 +/- 0.7 (+/-sd) yr] had lower aBMD than those with MENA less than the median (EARLIER: 12.1 +/- 0.7 yr) in total radius (P = 0.026), diaphysis (P = 0.042), and metaphysis (P = 0.046). LATER vs. EARLIER displayed lower total volumetric BMD (315 +/- 54 vs. 341 +/- 56 mg HA/cm(3); P = 0.010), cortical volumetric BMD (874 +/- 49 vs. 901 +/- 44 mg HA/cm(3); P = 0.003), and CTh (774 +/- 170 vs. 849 +/- 191 microm; P = 0.023). CTh was inversely related to CSA (R = -0.46; P < 0.001). In LATER reduced CTh was associated with 5% increased CSA. CONCLUSIONS: In healthy young adult women, a 1.9-yr difference in mean MENA was associated with lower radial aBMD T score, lower CTh without reduced CSA, a finding compatible with less endocortical accrual. It may explain how late menarche is a risk factor for forearm osteoporosis.