demoOnly1 is one of the the latest demos of Brutal Mario. It only has barely a few playable levels, which are Athletic Special, Baby Yoshi Hill, Lemmy's Castle, Lively Water, all the four Switch Palaces, Journey's Beginning, Yoshi's House, Frozen Cave and maybe others. The majority of the levels freeze on the "Mario Start" screen, have extremely glitchy graphics or have non-solid ground that Mario passes through and dies. Athletic Special is the only new stage and the Lemmy Boss has been remade. The level titles are also screwed up, as the ones outside of the Special Zone appear as jumbled letters. This is because of the added English font, which replaces all of the Japanese symbols.

It is prequel to the 1-1 Demo and the sequel to the Boss Demo.