World A is the second secret world and the tenth world that was first featured in the game Super Mario Bros.: The Lost Levels, coming after World 9. The Koopa Troopas from this world on move faster than the normal ones.
In the Famicom Disk System version of Super Mario Bros. 2, the player must beat the game eight times and press the Start button while holding down the A button on the title screen in order to access World A, and in turn the other later worlds. In the Famicom Mini version for the Game Boy Advance, this was changed so that the B button must be held instead of the A button. However, in the Super Mario All-Stars version of the game, the player proceeds to the later worlds directly after completing World 8 (or World 9 if the player accesses it by beating Worlds 1 to 8 without using warp pipes). World 9 and the later worlds were left unused in the Super Mario Bros. Deluxe version of the game.