The investigators second hypothesis is that loss of heterozygosity (LOH) detected bij DNA markers is a reliable indicator for progression to malignancy. By intensifying the outpatient clinic follow up and treatment in premalignant lesions, the incidence of progression to invasive carcinoma is expected to be significantly reduced. If this hypothesis is justified, there will be a place for CIN and LOH detection as a risk indicator in the diagnostic work up of premalignant lesions in the head and neck.