Early and accurate diagnosis of emphysema is said to be invaluable for identification of clinically silent and mild emphysema. Recently, computed tomography (CT) has been much advocated for its efficacy in detailed diagnosis of emphysema, and the results have been compared with the pathology grade of emphysema in resected lung specimens. To assess the ability of high resolution CT scan in detecting and grading mild emphysema, we correlated the high resolution CT scan with the pathology grade of emphysema and the destructive index (DI) of lung specimens from 42 patients undergoing thoracotomy for a solitary pulmonary nodule. The high resolution CT scan and the cut surface of the lung, corresponding exactly to the same plane of the CT scan image, were assessed using the picture-grading system of Thurlbeck and coworkers on a scale of zero to 100. The CT scores for all patients ranged from 12 to 57, with a mean +/- SD of 22.1 +/- 9.6 using 1-mm collimation (n = 35), and from 7 to 46 with a mean +/- SD of 16.5 +/- 8.3 using 5-mm collimation (n = 33). The pathology scores ranged from 10 to 57, with a mean +/- SD of 23.2 +/- 9.8 (n = 42). The DI ranged from 15.4 to 67.1, with a mean +/- SD of 31.4 +/- 10.8 (n = 42). The CT scores using 1-mm and 5-mm collimation correlated significantly with the pathology scores (r = 0.68 and 0.76, respectively, p less than 0.001), and with the DI (r = 0.62 and 0.74, respectively, p less than 0.001). The pathology scores correlated significantly with the DI (r = 0.72, p less than 0.001). We therefore concluded that high resolution CT can help to identify the presence and grading of mild emphysema.