A 20-month-old girl died in renal failure with macrobullous medullary polycystic disease of the kidney and bilateral congenital cystic disease of the lungs. Respiratory difficulty and a shifted mediastinum necessitated two pulmonary resections in infancy. The surgically resected cysts were clearly malformations consistent with cystic adenomatoid malformation and were entirely different from the cysts present at necropsy which resembled dilated normal bronchioles. The macrobullous renal cysts were confined to the medulla; however, the clinical course and the morphologic appearance of the cystic kidney were typical of neither medullary sponge kidney or medullary cystic disease. Recent evidence suggests that the differences between medullary sponge kidney, medullary cystic disease, and polycystic kidney, in some instances, may be a result of clinical selection. We concluded that the association of macrobullous medullary polycystic kidney disease and pulmonary cysts in this patient was fortuitous.