Bring pineapple and sugar to a boil in a large saucepan and cook 20 minutes, stirring constantly until thickened or until the temperature reaches 220F on a thermometer (this is the jelling stage at sea level).
Pour immediately into hot sterilized jars; wipe rims with a clean cloth, place lids and screw on bands fingertip-tight.
Process jars in a water-bath canner for 5 minutes.
Place jars on a clean towel.
Cover with towel to prevent drafts and let cool undisturbed for 24 hours.
