A 1974-vintage 25 cm field of view gamma camera was converted to a bone mineral densitometer using a purpose-built converging collimator with attached C-arm holding a 250 mCi gadolinium (Gd) 153 source. After addition of a second pulse-height analyser, increasing the high voltage gain and connecting to a digital computer, spinal and hip bone mineral density measurements were made. Accuracy of 2% and long-term reproducibility of 2.7% were obtained using an anthropomorphic bone phantom. For lumbar spine measurements in normal volunteers, long-term reproducibility was 3.2% and for osteoporotic (spinal fracture) patients (mean density 831 mg cm-2) reproducibility was 3.7%.