A variety of mediastinal masses contain areas with CT attenuation values lower than that of chest wall musculature and higher than that of subcutaneous fat. In some cases, the low attenuation is not absolute but is relative to the enhancing wall of the lesion or to the surrounding structure on contrast CT images. In these cases, precise determination of the attenuation values discloses a density equivalent to that of soft tissue. The authors retrospectively analysed the CT findings of 28 cases of pathologically proven mediastinal masses containing low-attenuation areas. Thymic lesions, germ cell tumors and intrathoracic goiters were mostly in the superior portion of the anterior mediastinum, but neurogenic tumors and forgut cysts were the largest masses. However, there was no specific relation in size of the other masses. Foregut cyst showed the lowest attenuation density and thymic lisions and intrathoracic goiters showed relatively higher attenuation than other lesions. Through analysing the size, location, shape, density and enhancement effect in pre-and post-contrast studies, CT is helpful in differentiating Low-Attenuation Mediastinal Masses.