A high degree of uncertainty and irritation predominates in the assessment and comparison of radiation dose values resulting from measurements of bone mineral density of the lumbar spine by photon absorptiometry and X-ray computed tomography. The skin dose values which are usually given in the literature are of limited relevance because the size of the irradiated volumes, the relative sensitivity of the affected organs and the radiation energies are not taken into account. The concept of effective dose, sometimes called whole-body equivalent dose, has to be applied. A detailed analysis results in an effective dose value of about 1 microSv for absorptiometry and about 30 microSv for computed tomography when low kV and mAs values are used. Lateral localizer radiographs, which are necessary for slice selection in CT, mean an additional dose of 30 microSv. Lateral X-ray films of the spine which are frequently taken in combination with absorptiometry result in a dose of 700 microSv or more. The concept of effective dose, the basic data and assumptions used in its assessment and a comparison with other dose burdens (for example the natural background radiation, of typically 2400 microSv per year) are discussed in detail.