The Balloon Boo is a subspecies of Boo that appears in New Super Mario Bros. on the Nintendo DS. Appearing only in the World 4 Ghost House, these Boos can be recognised by their strained facial expressions and pink cheeks. Their tails also resemble the nozzle of a balloon.
Like typical Boos, Balloon Boos will not attack when they are being looked at. However, when Mario or Luigi looks away, the Balloon Boo will expel air from its mouth and deflate as it floats toward its quarry. It will continue chasing even when reaching minimum size, but when the player looks at it again, it will stop moving and re-inflate slowly. While inflating up again, the player will have the chance to take advantage of its smaller size and jump over or run under the Balloon Boo depending where it is on the screen. The Balloon Boo will not chase the player beyond the room it's in. Also, Balloon Boos cannot be defeated by any power-ups other than Mega Mushrooms or Blue Shells.