Luigi's Mansion/The Hidden Mansion

After you complete the game, you will return to Professor E. Gadd's laboratory. When he asks you where you want to go, you'll notice a new option, the "Hidden Mansion". This is essentially the game's "hard mode". The Poltergust is about 1.5 times more powerful, but ghosts put up more of a fight to escape its pull. There are more powerful ghosts in earlier rooms: for example, the invisible Grabber ghosts first appear in the Wardrobe Room, but appear much later in the Normal Mansion. Because of all this, there is much more treasure to be found in the Hidden Mansion, making it more rewarding, and easier to achieve a Rank A mansion at the end.

PAL version differences
The PAL version of the Hidden Mansion has been altered to increase difficulty. Some of these changes were added by Shigeru Miyamoto himself.
 * In the Hidden Mansion, the entire mansion is mirrored. So, for example, in the Foyer, the Toad on the left is now on the right.
 * Certain bosses' attacks are made harder. For example, Chauncey's rocking horses are more difficult to avoid, since they zig-zag towards Luigi, and travel faster than in the Normal Mansion.
 * During the Boolossus battle, when trying to 'pop', him, you must ride around on the actual vacuum itself, making controlling Luigi harder.
 * The rooms are significantly darker when compared to the Normal Mansion.
 * 45, rather than 40 Boos are required to access the final boss.
 * The amount of cash required to achieve Rank A at the end is increased, so the only way to do so is in the Hidden Mansion, where more cash is available.