Modern computed tomographic (CT) scanners have post-processing facilities which allow pixels within a selected density range to be highlighted. It has been suggested that using this technique to highlight regions of low CT density (-900H to -1000H) can locate and quantitate areas of pulmonary emphysema. To establish the variability of the lung density map in normal subjects this technique was used in 17 patients with no evidence of structural lung disease on full respiratory function testing. Mean CT lung density varied from -770H to -875H (overall mean -817H). The percentage cross-sectional area in the low density range -900H to -1000H varied from 0.6% to 58% (mean 15.3%). In two further groups of 12 subjects the influence of CT slice thickness and intravenous contrast administration on the lung density map was evaluated. The use of narrower collimation led to an increase in mean cross-sectional area within the low density range from 9.6% to 16.1% (P less than 0.05). The use of intravenous contrast led to a reduction in mean cross-sectional area within the range from 8.9% to 3.3% (P less than 0.01). There is considerable variability in the lung density map of normal individuals and the method is also dependent on radiographic technique. These factors should be taken into account when considering the use of CT lung density mapping for the assessment of pulmonary disease.