Three patients developed late pulmonary vein obstruction (PVO) following Mustard's operation for transposition of the great arteries. In all three the absence of PVO had been documented by an earlier postoperative cardiac catheterization. At reoperation shrinkage and kinking of the dacron baffle was evident. After Mustard's operation, patients should be observed closely for symptoms and signs of PVO since it can occur insidiously despite previously proven absence of this complication.