He was the host of the weekly American Top 40 radio show from 1970 to 1988 , and again from March 1998 until January 10 , 2004 . Ryan Seacrest is now the host . Kasem is best known as a music historian and disc jockey , most notably as host of the weekly American Top 40 radio program from July 4 , 1970 to 1988 , and again from March 1998 until January 10 , 2004 , when Ryan Seacrest succeeded him .