In a skillet over medium heat, brown sausage; drain. In a large bowl, combine sausage, hashbrowns, mushrooms and cheese. Spread evenly in a greased 13"x9" baking pan.
Beat together eggs, milk, salt and pepper. Pour egg mixture over hashbrown mixture.
Cover with aluminum foil and bake at 375 degrees for one hour. Remove foil and continue baking for an additional 15 minutes, or until golden.