To examine at which stage in the multistep process of head and neck tumorigenesis numerical chromosomal alterations can be detected by fluorescence in situ hybridization (FISH), biopsies and cell smear preparations of clinically healthy oral tissue, premalignant lesions (leukoplakias), and tumors were analyzed by FISH using chromosome-specific centromeric probes. Aberrations found in tumor biopsies and in tumor cell smears consisted of trisomy of chromosomes 1, 7, 10, and 17 and monosomy of chromosomes 1, 7, 9, 10, and 17. In five of eight dysplastic oral leukoplakia biopsies, aberrations were seen consisting of trisomy of chromosome 1, 7, and 17, and monosomy of chromosome 9. No aberrations were found in biopsies of hyperplastic lesions (n = 8), or in oral cell smears of persons at risk. Because numerical chromosomal aberrations seem to be highly specific for malignant cells, FISH may help to identify leukoplakias that have a high risk of malignant conversion.