Place potatoes in 8-10 quart saucepot; add water to cover; bring to boil over high heat.
Lower heat to simmer and cook, covered, 15-20 minutes, or until tender.
Heat oven to 325 degrees F.
Drain potatoes thoroughly, then mash until smooth.
Beat in butter, milk, pineapple, salt, and pepper.
Spoon into 9"x13" baking dish or 3 quart casserole; top with marshmallows.
Bake for 15-20 minutes until marshmallows have lightly browned.
Note: Casserole may be prepared in advance and set aside or refrigerated until baking time; in that case, baking time must be increased so potatoes will heat through, and marshmallows should not be added until last 15 minutes of baking time.