The present study investigated the potential of dual energy X-ray absorptiometry to determine the effect of dietary protein on live pig body composition compared with the standard methods. The experiment utilised 48 Large White × Landrace gilts, stratified on liveweight at 17 weeks of age (initial liveweight about 60 kg), and allocated within strata to diets containing either 85 or 100% of dietary protein requirements. The diets were fed ad libitum and contained 14.6 or 16.7% protein, respectively. Pigs were housed individually to allow weekly measurement of feed intake and liveweight. A Hologic QDR4500A dual energy X-ray absorptiometer was used to determine lean, fat and ash composition of pigs initially and 4 weeks later at the end of the experiment. Daily gain was increased by about 200 g in pigs fed the protein-adequate diet, but feed intake was not affected. Therefore, pigs fed adequate protein had a lower feed conversion ratio (2.92 v. 3.52 g/g, P<0.001). Feeding the protein-adequate diet increased lean deposition by about 150 g/day (577 v. 429 g/day, P<0.001) and ash deposition by about 4 g/day (28.7 v. 25.0 g/day, P<0.001), but did not affect fat accretion. This resulted in carcasses with a higher lean content (54.0 v. 50.7 kg, P<0.001) and ash content (2.35 v. 2.28 kg, P<0.05) but unaltered fat content. Therefore, an inadequate level of dietary protein leads to suboptimal growth in lean tissue and bone mineral, with no pronounced effect on fat. These observations were substantiated by chemical analysis and available corrective equations were useful in correcting differences between dual energy X-ray absorptiometry outputs and chemical values. After correction, dual energy X-ray absorptiometry estimates differed less than 5% from the chemical values for lean, protein, water and ash and 10% for lipid. In addition, reduced standard error of the differences around most dual energy X-ray absorptiometry measurements, relative to chemical analysis, allowed small changes in body composition to be detected with increased confidence. These data support the efficacy of using dual energy X-ray absorptiometry in nutritional studies of pigs.