The lack of consensus of how the results of peripheral bone mineral density (BMD) measurements should be interpreted is proving a barrier to the wider use of these devices. One approach is to interpret peripheral measurements using thresholds (so-called equivalent T-scores) defined to have the same absolute fracture risk as a femoral neck T-score of -2.5. For this concept to be valid, the estimates of fracture risk for a population should be the same irrespective of the measurement technique used. We tested this prediction both theoretically and in vivo using data for 63 postmenopausal women with Colles fracture and 191 control subjects. The theoretical analysis showed that if the normal population has a Gaussian BMD distribution and fracture risk varies exponentially with Z-score as exp(-beta Z) then patients who experience a low-trauma fracture have a fracture risk that is larger by a factor exp(beta(2)) compared with the fracture risk of the whole population. Using data from the in vivo study, fracture risk predictions were compared for seven different types of measurement (lumbar spine; femoral neck; total hip BMD; and speed of sound [SOS] at the radius, tibia, phalanx, and metatarsal). When quantitative estimates of fracture risk were made for individual subjects, the average risk of fracture for the Colles group varied between 1.03 times larger (for tibial SOS) and 2.77 times larger (for total hip BMD) than the average fracture risk for the whole population. As predicted by the theoretical study, fracture risk varied according to the odds ratio determined by logistic regression analysis. Therefore, estimates of fracture risk derived for the same group of patients varied almost threefold according to the type of measurement. It was concluded that equating estimates of absolute fracture risk for different types of scan should not be used as the basis of deriving equivalent T-scores for interpreting peripheral measurements.