An investigation into the possible effect of fat on the estimation of trabecular bone mineral concentration by computed tomography (CT) is described. Computed tomographic attenuation and effective atomic number were measured in models simulating trabecular bone, and in cadaveric femoral using a simultaneous dual energy technique. The composition of the area of bone scanned was measured by chemical analysis. In the bone models, CT attenuation value and effective atomic number were positively correlated to the calcium concentration; fat modified CT attenuation value but not atomic number. The calcium concentration of the trabecular bone samples was positively correlated with effective atomic number (r = 0.972; p less than 0.001) and to a lesser extent with CT attenuation value (r = 0.67; p less than 0.01), but both relations were independent of fat. Under the conditions of these experiments, the determination of effective atomic number by dual energy CT provides a more accurate measure of trabecular bone mineral concentration than does CT attenuation value by a single energy technique. There was a considerable spatial variation in the amount of bone tissue in the trabecular bone space, and CT attenuation values varied with the site and size of the area of interest. Accurate registration is essential for the precise measurement of trabecular bone mineral by CT.