Population based studies have demonstrated that having a first degree relative with a hip fracture is predictive of future hip fractures. Postmenopausal bone mineral density (BMD), ultrasound of calcaneus and hip axis length are associated with hip fracture, with the association for ultrasound and hip axis length being independent of BMD. The aim of this study was to determine the genetic component of these three important risk factors. We performed a classical twin study using 500 normal female twins, 128 identical and 122 non-identical pairs, aged 50 to 70 years. We measured bone mineral density at multiple sites, hip axis length (distance from the inner rim of the acetabulum to the greater trochanter), broadband ultrasound attenuation and velocity of sound of the calcaneus. Bone density had a strong genetic component at all sites with estimates of heritability ranging from 0.46 to 0.84. Hip axis length and velocity of sound had major genetic components with estimates of 0.62 and 0.61 respectively, which remained virtually unchanged after adjustment for bone mineral density. Broadband ultrasound attenuation had a moderate genetic component with an estimate of 0.53, which was reduced further to 0.45 after adjustment for BMD. In summary, all three bone measurements, which are independently associated with hip fracture, are independently heritable. This study suggests that a combination of different genetic factors acting on the structure, dimensions and density of bone may explain the importance of family history as a risk factor for hip fracture. (J Bone Miner Res 1996;11:53&534)