Sprinkle sugar over the rhubarb in a 9"x13" pan. Add marshmallows.

Mix 1 package of yellow cake mix according to the directions on package. (I use Duncan Hines lemon supreme cake mix.) Pour batter over rhubarb mixture. Bake 1 hour at 350 degrees. Cool a little and tip upside down on a cookie sheet or leave in pan and then upside down when you serve it. If you use frozen rhubarb it will take longer to bake.