Preheat the oven to 350 degrees F (175 degrees C).
In a large saucepan cook sweet potatoes in salted water over medium-high heat for 20 minutes, or until done. Drain, and add orange juice, brown sugar, nutmeg, and butter. Whip until smooth. Spread into a medium size casserole dish and top with marshmallows.
Bake in preheated oven for about 10 minutes, or until marshmallows are golden brown.