A study was made of the relation of pressure and volume for a cyst and bullae of the lung. Although the elastic properties of these abnormal structures of the lung were found to vary, their general relations were similar. A lack of elastic resistance was noted to be their property. The variation in their mechanical properties was associated with different degrees of intracystic pressures, which determine the amount of compression on the proximal lung tissue. Bullae and cysts can inflate but not deflate because their openings become compressed when they become inflated, and this factor maintains the state of inflation.