There is still some uncertainty in the assessment and comparison of patient dose which results from bone mineral measurements on the lumbar spine by photon absorptiometry and x-ray computed tomography (CT). In the literature, skin dose values are offered in almost all cases. These are not meaningful, however, as the size of the irradiated volume and the radiation sensitivities of the respective organs are not taken into account. Here, the concept of effective dose has to be applied. An assessment of effective dose resulted in approximately 1 mu Sv for absorptiometry and approximately 30 mu Sv for computed tomography with low kV and mAs values. For lateral lumbar spine films which are often taken in connection with absorptiometry, at least 700 mu Sv have to be added. In CT, additional 30 mu Sv result from the lateral topogram which is required for positioning. The concept of effective dose, the data and assumptions used in its assessment and a comparison to other risk factors, as for example natural background radiation of approximately 2400 mu Sv per year, are discussed in detail.