Bring sugar and water to a boil in a 3-quart heavy saucepan, stirring until sugar is dissolved. Add cranberries and simmer, uncovered, until berries are collapsed, 8 to 10 minutes. Pour through a large sieve into a bowl, gently pressing on solids to extract liquid (without forcing pulp through), and discard solids. Cool syrup, then chill, covered, until cold.
