Men exhibit age-related adaptive changes in long bone geometry, namely, endosteal resorption and periosteal apposition of bone, that help to preserve bone strength. It is not clear whether women undergo similar adaptive responses. To address this question, we assessed the bone mineral density and cross-sectional geometry of the radius and ulna at the one-third distal site by single photon absorptiometry and computed tomography (CT) in healthy young (n = 21, age 20-30 years) and older (n = 22, age 63-84 years) women. We used the CT data to compute the total subperiosteal, medullary, and cortical areas, as well as the maximum, minimum, and polar moments of inertia. We normalized the geometric parameters for bone length and performed comparisons using both the original and size-corrected data. Radial and ulnar bone mineral content and density were 20-30% lower in the older women (P < 0.0001). Ulnar width, total area, medullary area, and maximum and polar moment of inertia were greater in the older than in the younger women. Although we observed similar trends when we examined the radius data that were corrected for bone size, age-related differences in radial geometry were less pronounced and were not significant. We conclude that women undergo endosteal resorption and periosteal apposition of the ulna with age, thereby exhibiting an adaptive pattern that helps to preserve bone strength. The different behavior of these two bones suggests that local, rather than systemic, factors underlie this adaptation.