Mushrooms are primary power-ups that appear in all the Mario series. In the Mario Kart series Mushrooms give you a speed boost. During the other Mario games they make Mario grow. In the Mario RPG series they give you health.

In Super Mario Bros. Z, they're used twice in Episode 1 in order to boost karts, (one by Wario Bros. and the other is used by Mario Bros.) They have never been seen or used since.