The hypothesis is that chromosomal instability (CIN) detected by fluorescence is situ hybridization (FISH) is a reliable indicator for progression to malignancy. By intensifying the follow up and treatment in premalignant CIN 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 detection as a risk indicator in the diagnostic work up of premalignant lesions in the head and neck.