Effects of long-term exercise on volumetric bone mineral density (vBMD), bone mineral content, bone geometric properties, and the strength indexes of bone were examined in a cross-sectional study of athletes and controls. Tibias of 25 jumpers (13 women), 30 swimmers (15 women), and 25 controls (15 women), aged 18-23 yr, were scanned at midsite by using peripheral quantitative computed tomography. The cortical vBMD of female athletes was lower than that of the controls (2.00 +/- 0.05, 1.90 +/- 0.08, and 1.92 +/- 0.12 g/cm3, respectively, for controls, swimmers, and jumpers). On the other hand, periosteal areas of male jumpers and female athletes were greater than that of controls (460 +/- 50, 483 +/- 46, and 512 +/- 55 mm2, respectively, for male controls, swimmers, and jumpers, and 283 +/- 52, 341 +/- 73, and 378 +/- 75 mm2, respectively, for female controls, swimmers, and jumpers). The endocortical area of female swimmers was greater than that of controls (103 +/- 29, 148 +/- 52, and 135 +/- 54 mm2, respectively, for controls, swimmers, and jumpers). The polar moment of inertia and strength strain index of male jumpers and female athletes were significantly greater than those of controls, except for the difference in strength strain index between male jumpers and controls. We conclude that the improvement of mechanical properties of young adult bone in response to long-term exercise is related to geometric adaptation and not to vBMD.