In 1939 he had a serious illness and was operated on for a brain tumour. For some time he was not well enough to work. Then from 1947 to 1950 he returned to Europe to conduct the Budapest Opera. In the early 1950s Klemperer experienced difficulties arising from his U.S. citizenship. American union policies made it difficult for him to record in Europe, while his left wing views made him increasingly unpopular with the State Department and FBI. In 1952 the United States refused to renew his passport.