The purpose of this study was to determine the minimum thickness of cortical bone required for the accurate measurement of cortical material density by computed tomography (CT) and to establish normal reference values. A phantom with several wall thicknesses of bone-like material was constructed to simulate various cortical widths. The CT density at each level of thickness was measured on a GE 9800 CT scanner and on the OsteoQuant, a special CT scanner optimized for the measurement of bone in the extremities. The minimum width required to attain the correct material density was determined for each scanner. Additionally, the material density and width of the cortex in the radius and/or femur were measured by CT in 761 healthy subjects, ages 4-84 years. The minimum thickness necessary for an accurate density evaluation of the walls of the phantom by CT was 2-2.5 mm; below these thresholds the values fell in a linear way relative to width. In humans, the material density of cortical bone in the appendicular skeleton was not influenced by height or weight, and the values were similar for all subjects, as long as the cortical width was above 2-2.5 mm. The cortical width increased with age up to 30 years and decreased from 50 years on. We conclude that the material density of cortical bone in the appendicular skeleton can be measured accurately by CT if the thickness of the cortex exceeds 2-2.5 mm.