A dichromatic photon absorptiometric technique for the in vivo measurement of the bone mineral of the spine is discussed. A high purity, high activity153 Gd source which has photons of predominantly 44 and 100 keV was used as the transmission source. The transmission scans were performed on a modified Ohio Nuclear whole body rectilinear scanner. The precision of four measurements on three individuals and three measurements on one individual over a one month period was 2.3% (coefficient of variation). The accuracy determined by measurements of phantoms with known concentrations of dipotassium hydrogen phosphate solutions was 1.2% (standard error of estimate). This technique can be applied in most well equipped nuclear medicine departments.