Drain cherries, reserving 3/4 cup juice. Set cherries aside.
Combine juice, sugar, and flour in a medium saucepan. Cook over medium heat, stirring constantly, until thickened and bubbly. Remove from heat, and stir in cherries.
Spoon filling evenly into pastry shell. Arrange pastry strips in lattice design over filling; seal edges. Pastry strips may be brushed with milk to insure even browning. Bake at 375° for 45 minutes. Serve with vanilla ice cream, if desired.