The development of heart surgery lagged behind operations on other organs. In the 1920s surgeons in Boston and in Europe attempted to open mitral valves that had become obstructed as a complication of rheumatic fever. Most of their patients died, and the operation was abandoned until after World War II. Operations to treat children with specific types of congenital heart disease were developed between 1938 and 1944. But these procedures involved the blood vessels outside the heart rather than structures within it. After the war, surgeons in Boston, Philadelphia, and London showed that it was safe to operate on patients with severe mitral stenosis. Without surgery, these individuals would die of heart failure. Mid-century optimism about the potential of treating patients with heart disease was fueled by the discovery of so-called miracle drugs, such as penicillin and cortisone (for which two Mayo staff members shared the Nobel Prize in 1950).