They are Egyptian 100% cotton.  Flannel is warm for winter nights and with the heating blanket on low, everything is so cozy and warm that my son just fall asleep.  He had 50% cotton and 50% polyester sheets on his bed and he was sweating from the polyester in the sheets.  He want nothing but 100% cotton from now on.  They are a beautiful plaid sheets.  We will purchase more 100% cotton sheets.