Prepare a water-bath canner, jars, lids, and rings. Keep the jars hot until ready to fill.
Wash and juice the lemons. Strain out the seeds and excess pulp.
Combine the sugar and water in a large stainless steel saucepan.
Heat over medium heat, stirring until the sugar completely dissolves.
Add the strained lemon juice and heat the mixture thoroughly. Do not boil it down.
Ladle the hot lemonade concentrate into hot jars, leaving ΒΌ inch of headspace.
Remove air bubbles, check the headspace, and wipe the jar rims.
Center the lids, add the rings, and tighten until fingertip tight.
Process pint or quart jars in a boiling water-bath canner for 10 minutes at elevations up to 1,000 feet. Adjust the processing time for your altitude.
Turn off the heat, remove the canner lid, and leave the jars in the canner for five minutes.
Remove the jars and let them cool undisturbed for 12 to 24 hours.
Check the seals, remove the rings, label the jars, and store them in a cool, dark place.