According to Wiesenthal's own recollections, he became a stamp collector in 1948, following the advice of his doctor. Apparently, the work of tracking down hidden Nazi criminals and collaborators was having a negative impact on his health. To alleviate his severe insomnia, his doctor suggested a relaxing hobby like philately.
As a passionate stamp collector, he also collected stamps related to the Holocaust. For example, stamps issued in ghettos or commemorative stamps issued after the war to mark the anniversaries of the liberation of the camps.