Upper aerodigestive tract tumorigenesis has been hypothesized to represent a field cancerization process with multistep events based on its association with known carcinogens, its frequent associated premalignant lesions, and its multifocal clinical manifestation. To further explore this working hypothesis, we have examined normal tissue and premalignant lesions in the field of tumors for evidence of genetic change. Paraffin sections of head and neck tumors harboring neighboring premalignant lesions were explored for the presence of chromosome polysomies using in situ hybridization and chromosome-specific centromeric probes. Cells exhibiting random polysomy were observed in the premalignant regions near the tumors. The frequency of polysomy in the tumor field increased as the tissue progressed from normal morphology (33%), to hyperplasia (67%), to dysplasia (95%), and to squamous cell carcinoma (96%). These results support the notions of field cancerization and multistep tumorigenesis in the aerodigestive tract. To determine whether the degree of accumulated genetic alterations might serve as a biomarker for risk of developing malignancy, a set of biopsies of oral premalignant lesions (leukoplakia, erythroplakia) were retrospectively chosen for polysomy analysis from two groups of individuals: one group who subsequently developed oral cancer and one group who did not develop oral cancer. Three of the five individuals who showed significant chromosome polysomies in their biopsies subsequently developed oral cancer, whereas only one of eight individuals with little evidence of polysomy subsequently progressed to oral cancer. These results suggest that evidence of generalized genetic change or instability might be useful as a genetic biomarker for risk assessment.