Place pears in a food processor or blender; puree until smooth.
Measure 3 cups pureed pears into a saucepan. Add sugar. Cook, stirring occasionally, until bubbling, about 15 minutes. Stir in cinnamon. Cook, stirring frequently, until thick, about 15 minutes more.
Immerse a 1-pint jar in simmering water for 5 minutes. Wash lid and ring in warm soapy water. Fill hot jar with pear mixture. Seal and store in the refrigerator.
