The Variable Composition Phantom or VCP (Lunar Corp., Madison, WI) consists of acrylic blocks and thin sheets of PVC and vinyl used to simulate five different percent fat values. The total weight is about 10 kilograms. The Whole Body Phantom or WBP (Hologic Inc., Bedford, MA) is larger than the VCP and has aluminum embedded in it to simulate the skeleton. It consists of six rectangular blocks of highdensity polyethylene and a sheet of PVC. It weighs about 43 kg. We scanned both phantoms and 17 subjects on two QDR 4500A densitometers (scanner A and B). Short-term precision of the phantom measurements was determined by scanning the phantom 10 times successively.