Place tomatoes in a flat-bottomed kettle. Slowly bring to a boil. Reduce heat; cover and simmer 30 minutes. Add lemon slices and sugar, stirring well. Bring to a boil; boil until mixture registers 220° on a candy thermometer or until mixture sheets from a cold metal spoon. Remove from heat, and skim off foam with a metal spoon.
Quickly ladle marmalade into hot sterilized jars, leaving 1/4- inch headspace. Cover at once with metal lids, and screw bands tight. Process marmalade in boiling-water bath 10 minutes.