Heat oven to 200°F. Grease 2 cookie sheets. In heavy 5- to 6-quart saucepan, place sugar, corn syrup and water. Heat to boiling over medium heat, stirring frequently. Stir in butter.
Cook over medium heat, stirring occasionally for about 10 minutes until candy thermometer reaches 240°F. Meanwhile, place cookie sheets in oven. (Warm cookie sheets allow the candy to spread easily before it sets up.)
Stir pistachios into sugar mixture; continue cooking, stirring frequently, until candy thermometer reaches 300°F. Remove from heat; stir in baking soda (mixture will be light and foamy).
Remove cookie sheets from oven; pour candy onto cookie sheets. With buttered spatula, spread until candy is about 1/4-inch thick. Cool completely, about 1 hour. Break into pieces. Store in tightly covered container.