1. There are few data regarding the accuracy of Hologic QDR-1000W dual-energy X-ray absorptiometry for the measurement of body composition. In two studies, one in an in vitro experimental system using oil and water mixtures and the other in samples of pork meat, the effect of depth and tissue thickness on the measured composition was assessed. In the latter study the measured fat mass was compared with that measured by direct analysis. 2. All data indicated a trend in the measured fat mass with depth, such that more fat was measured at extremes of depth (< 10 cm and > 25 cm) than at intermediate depths. 3. In samples of meat weighing approximately 55 kg, dual X-ray absorptiometry significantly under-estimated the absolute fat mass compared with direct analysis (mean 20.4 +/- 1.65%) by 5-8% or 1-4 kg of fat. 4. These findings are of direct relevance to both clinical and research work using this technique to measure body composition, in particular in circumstances in which changes in body composition and/or tissue thickness are anticipated.