Save the syrup to sweeten tea.
Cut lemons into 1/8-inch-thick slices; discard seeds. Stir together water and sugar in a large, heavy saucepan. Bring to a light boil, stirring just until sugar dissolves. Add lemons; reduce heat to medium-low. Gently simmer 45 minutes or until rinds are very soft and lemons are translucent, turning lemons every 15 minutes. Remove with a slotted spoon, and place in a single layer on a wire rack. Cool completely.