The percentage of the low attenuation area (LAA%) is understood to be the most accurate index in quantifying emphysema by computed tomography (CT). To date, CT scanners with enhanced post-processing programs have been used for measurements of LAA%. In this study, we sought develop a method of LAA% determination using a conventional density mapping program for CTs and NIH Image, an image-analysis software package for personal computers. Our results for overall LAA%, together with the correlations between overall LAA% and pulmonary function tests, agreed well with the findings of earlier reports. In reproducibility trials utilizing our method interoperator error for LAA% was within +/- 2%. A phantom experiment indicated that there was little difference between LAA% values obtained by this method and the values obtained by direct CT measurements. Therefore, we concluded that this method is sufficiently reliable for clinical use and capable of facilitating LAA% measurements with CT scanners that are not equipped with enhanced post-processing software.