1. Preheat oven to 450 degrees F (230 degrees C).
2. Combine sugar and flour. Sprinkle 1/4 of it over pastry in pie plate. Heap rhubarb over this mixture. Sprinkle with remaining sugar and flour. Dot with small pieces of butter. Cover with top crust.
3. Place pie on lowest rack in oven. Bake for 15 minutes. Reduce oven temperature to 350 degrees F (175 degrees C), and continue baking for 40 to 45 minutes. Serve warm or cold.