From the Super Mario Wiki
The Golden Mushroom is an item that appears in several Mario games. It is a rare Mushroom, sometimes depicted with a crown on its head, and it always gives the player a positive effect when used. Golden Mushrooms sometimes vary by color and luster between games, but they are always a strong item.
 Super Mario World
 Mario Kart series
 Mario & Luigi series
In Mario & Luigi: Superstar Saga, it is used as a Mushroom and a Syrup in one item, restoring both HP and BP to max for a single brother. Though there are only five Golden Mushrooms in-game, a glitch that happens with Fawful or Popple and Birdo can net the player with an infinite amount of them. The player must use Swing Bros. and get the mushroom then get a game over and return to the place. The player keeps the golden mushroom, and can repeatedly do that as long as they wish.
In Mario & Luigi: Partners in Time, the Mushroom Drop is also golden and is the games Nut. They heal the Mario Bros. and the Baby Mario Bros. 15, 30, and 60 HP for the normal, Super, and Ultra versions, respectively. Although they aren't called golden mushrooms, their appearance is very similar to that of most of the golden mushrooms.
 Mario Party series
In Mario Party 2, 3, 5, 6 and 7, it allows players to roll three Dice Blocks at one time on their turn, (or in Mario Party 5, on their next turn if they landed on a space containing it), instead of just the standard one Dice Block. In Mario Party 4, it is replaced by Super Mega Mushroom, which serves a similar purpose. In Mario Parties 2-7, the user gets 30 coins for a triple number and a 50 coin bonus for triple 7s.
Mario Party 3 features the Golden Mushroom as Mario's favorite item.
 New Super Mario Bros. 2
Golden Mushrooms appear in New Super Mario Bros. 2 in the Coin Rush mode and replace 1-Up Mushrooms, and when Mario collects it, he gets 50 coins (or 100 coins if he is Gold Mario) upon contact. Its appearance is more metallic and lacks the crown from its Mario Kart appearances.
 Mario Kart Wii Trading Card Description
Golden Mushrooms generally show up in the Item Windows of racers near the back of the pack. It's an awesome tool for those unfortunate enough to be in that position as they can use it to catch up to those in front. Unlike other Mushrooms, however, you can tap the Item Button as many times as you'd like to maximize your boost. Be sure to cut corners where possible with this item.
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