PURPOSE: We retrospectively reveiwed the pathologic changes and Hounsfield unit of the low density lesion in solid mass on CT. MATERIALS AND METHODS: Pathologically proved solid mass was evalualed in regard to the shape and margin of the low density in the mass on the CT scans of 23 patient. The CT number of the low density lesion was correlated with the pathologic changes. RESULTS: Pathologic changes of the low density lesions were ;necrosis (n=17), hemorrahage (n=13), cyst (n=4), myxoid degeneration (n=2), hyaline degeneration (n=l), fibrosis (n=l), and mixed cellularity (n=l). In 14 cases, more than 2 pathologic changes were seen. In 11 cases, necrosis was associated with hemorrhage. The CT number ranged from 11.5 to 44.9 Hounsfield units(HU) (mean, 25.2HU). The average CT number was 26.9HU in hemorrhage and necrosis, 17.2HU in cystic change, 20.9HU in myxoid degeneration, 35.7HU in hyaline degeneration, 22.3HU in fibrosis, and 21.4HU in mixed cellularity. The hemorrhage and necrosis in 17 cases showed irregular margin, amorphous shape, and showed centrifugal distribution. The cystic change in 4 cases showed well defined margin, round shape, and peripheral location in solid mass. CONCLUSION: The low density lesions in solid mass on CT represented variable pathologic changes; necrosis, hemorrhage, cyst, myxoid degeneration, hyaline degeneration, fibrosis, and mixed cellularity. Pathologic changes would not be differentiated on the basis of CT number.