Published articles on the radiologic aspects of Proteus syndrome are sparse. This report highlights the features of this disease with specific attention to the serious pulmonary manifestations that may occur at an early age. Two cases of Proteus syndrome and severe lung disease are presented, with complete autopsy in one case and correlative surgical pathologic data in the other. Multiple superficial and visceral vascular abnormalities were present in both cases. Both patients developed rapidly progressive diffuse cystic emphysematous pulmonary disease leading to the death of one patient at age 18 years and a heart-lung transplant in the other at age 8 1/2 years. Extensive gross pulmonary cysts were evident pathologically with diffuse panlobar emphysema microscopically. Studies of collagen and cultured fibroblasts in one patient revealed no abnormality. The early presentation, rapid progression and potentially lethal nature of lung involvement has not previously been emphasized.