A Big Mushroom is an item introduced in Generation II. This item can be bought for 
5000 and sold for 2500.
They are mainly held by wild Paras and Parasect 5% of the time excluding the Hoenn region games. Or 8% of the time in Generation II. These items don't do much, but only have an effect in Pokémon FireRed and LeafGreen on where the player needs to exchange two of these to the Move Relearner to allow his/her Pokémon to remember a move.
After Generation III, they were found in various locations. Later in the Generation V games, Foongus and Amoonguss started holding these items with a 5% chance, like Paras and Parasect did.
This article is a stub!
Please help the Pokémon Wiki by adding more info.