In a heterogeneous group of 12 male and 30 female patients, total body calcium (TBCa) was measured by neutron activation analysis, and bone mineral content of the distal femur and six regions of the distal radius and ulna were measured by photon absorptiometry. TBCa was an average of 71 g (9%) lower than normal in the females and 119 g (11%) lower than normal in the males. Of the forearm measurements, those of the radial shaft provided the best estimate of TBCa with a standard error of estimation (SEE) of 88 g (14% of the mean value) for female patients alone, and 113 g (16%) for the whole group. The measurement on the femur provided a better estimate with SEE 80 g (13%) for female patients and 90 g (13%) for the whole group. These SEE are comparable with the variability in TBCa between normal subjects, which has a coefficient of variation of the order of 20% in both sexes. A slightly better estimate of TBCa can be obtained by using a multiple regression equation involving both the femur and radial shaft measurements with a SEE of 74 g (12%) for female patients and 81 g (11%) for the whole group. The correlation between the estimated deficit of TBCa and the estimated deficit of bone mineral from the forearm and the femur has been calculated.