1. We have examined the relationship between the attenuation of broadband ultrasound in the os calcis in vitro and its bone mineral density measured by quantitative computed tomography and by physical density. 2. Broadband ultrasound attenuation was found to correlate closely with physical density (r = 0.85, P less than 0.0001), but the correlation was less than that observed between quantitative computed tomography and physical density (r = 0.92, P less than 0.0001). Measurements of broadband ultrasound attenuation and quantitative computed tomography were significantly correlated (r = 0.80, P less than 0.0001). 3. Partial correlation analysis showed a significant relationship between broadband ultrasound attenuation and bone density, but when the effect of physical density was taken into account no significant correlation was found between broadband ultrasound attenuation and quantitative computed tomography (r = 0.08, not significant). 4. Broadband ultrasound attenuation in three prospective amputees showed a high degree of concordance between measurements in vivo and in vitro, with no interference by surrounding soft tissues. 5. The correlation between physical density and broadband ultrasound attenuation was independent of quantitative computed tomography, suggesting that the technique measures aspects of density which differ from its mineral density. Broadband ultrasound attenuation holds promise as a reproducible, rapid, radiation-free assessment of skeletal status.