Cystic transformation of the lungs in Proteus syndrome is considered an important manifestation of this disease. We describe a case of an 11-year-old girl with a diagnosis of Proteus syndrome with lung involvement. Low-dose multidetector computed tomography (CT) revealed extensive diffuse cystic lung disease with left lung predominance, affecting mostly the lower lung zones. The cystic lesions had various sizes and variable wall thickness. Postprocessing using CT histogram densitometric volumetry software (CT densitovolumetry), using the threshold −950 Houndsfield units (HU) for quantifying emphysema, revealed that 31% of her total lung volume was composed of areas with CT attenuation values below −950 HU.