Remove stems from mushrooms, chop stems, set mushrooms and stems aside.
In a 10 inch skillet over medium heat, cook sausage until well browned. With slotted spoon, remove sausage to paper towels and drain. Spoon out all but 2 Tbsp drippings from skillet.
In hot drippings over medium heat cook mushroom stems until tender, about 10 minutes, during the last 3 minutes add the pepperoni.
Remove skillet from heat and stir in sausage, cheese and crumbs.
Preheat oven to 450 degrees, fill mushroom caps with sausage mixture, place stuffed mushrooms into oven proof dish and bake 15 minutes.
VARIATIONS; Add chopped onions, chopped green peppers, bacon bits, anything you like in your pizzas.