Following his retirement in 1957, Jeppesen resided in Italy, enabling him to make several discoveries in Italian libraries culminating in his magnum opus, "La frottola" (1968–70), a detailed study and bibliography of frottole, the leading genre of Italian popular, secular songs in the late fifteenth and early sixteenth century. From 1927 until his death he was active in the International Musicological Society, serving as president from 1949 to 1952. He was also a member (from 1963) of the Italian Accademia dei Lincei.