To make the syrup, combine the basil, sugar and 1 cup water in a saucepan. Simmer and stir over medium heat until the sugar is dissolved, about 5 minutes. Cool and strain. Set aside or store in the refrigerator until ready to use.
Mix the lemon juice, the basil syrup and sparkling water together in a pitcher. Store in the refrigerator until ready to serve. Pour over glasses filled with ice.