Arrange rhubarb evenly in bottom of a greased 1-quart casserole; sprinkle with cinnamon. Set aside.
Combine sugar and flour; cut in butter until mixture resembles coarse meal. Sprinkle sugar mixture evenly over rhubarb. Bake at 450° for 15 minutes. Reduce temperature to 375°, and bake an additional 25 minutes or until lightly browned. Spoon into serving bowls while warm, and top with ice cream.