Mystery Disc is a compilation album by Frank Zappa. It was released on CD in 1998, compiling tracks that were originally released on two separate vinyl records and included in the mail order Old Masters box sets, which were released in three volumes between 1985 and 1987. (These box sets, issued on Barking Pumpkin, contained repressings of Zappa's albums from Freak Out! (1966) to Zoot Allures (1976), along with a 'Mystery Disc' in boxes one and two.) The CD omits the last two tracks from the 1985 LP, "Why Don'tcha Do Me Right?" and "Big Leg Emma", both of which were included on the CD version of Absolutely Free (1967) in 1989.