Quantitative computed tomography (QCT) for measuring bone mineral content of lumbar vertebrae is increasingly used internationally. The effect of using conventional CT (single energy CT, SE-CT) and dual energy CT (DE-CT) on reproducibility has been examined. We defined a standard measurement protocol, which automatically evaluates a calibration phantom. This should ensure an in vivo reproducibility of 1 to 2%. Reference data, which has been obtained with this protocol from 113 normal subjects, using SE-CT and DE-CT, are presented.