A method was developed for measuring total body bone mineral (TBBM) and lean body mass in vivo using dual-photon absorptiometry. The entire body was scanned in a rectilinear raster (transverse speed of 1 cm/s and longitudinal steps of 2.5 cm) with a modified nuclear medicine scanner and conventional nuclear counting electronics. The source was 153Gd (1 Ci) with principal photopeaks at 44 and 100 keV. The scan time was about 70 min with an absorbed dose of under 1 mrem. The low dose allows measurements to be repeated at frequent intervals or used on children. Short-term (months) precision of TBBM was about 1.5% for isolated skeletons and about 2% on normal human subjects. Long-term (years) precision on skeletons was under 3%. The precision of percent fat was 0.9%, which would lead to an error of less than 1% in the TBBM. Geometry of measurements also had minimal (and correctable) influence on the accuracy of results. The accuracy (1 standard error of estimate) of TBBM on isolated skeletons (N = 5) was 36 g (equivalent to about 13 g of Ca) with a correlation coefficient of 0.99; this error amounts to about 1--1.5% in normal adults; 2% in older women, and 2.5% in osteoporotic females. The dual-photon absorptiometry method could be implemented in many nuclear medicine departments to follow skeletal changes during growth and aging or to follow the course of a disease or treatment.