Purpose: Routine histologic examination cannot predict whether premalignant laryngeal lesions will progress toward invasive growth. The acquisition of changes in chromosome constitution has been suggested to be essential for driving tumor progression by enhancing mutagenic mechanisms. The aim of the present study was to determine whether chromosomal changes occur in the subsequent stages of early laryngeal carcinogenesis and, if so, whether these changes can be of prognostic value. Materials and Methods: Numerical aberrations for chromosomes 1 and 7 were detected in tissue sections from archival material using an improved in situ hybridization protocol. In total, eight benign laryngeal lesions, 37 premalignant laryngeal lesions, and 16 specimens containing histologically normal epithelia adjacent to laryngeal squamous cell carcinomas were studied, Both the histologic and the cytogenetic classifications were correlated with progression to laryngeal cancer, Results: No evidence for chromosome alterations was obtained in the control group, nor in histologically normal epithelia adjacent to laryngeal squamous cell carcinomas, nor in all bur one hyperplastic lesion (n = 11), In contrast, 14 of 15 dysplastic lesions and nine of 11 carcinomas-in-site contained numerical chromosomal aberrations, Tetrasomy was present in the majority of the dysplastic lesions, An unstable chromosome content (indicated by the presence of chromosome imbalances and/or polyploidization) in the premalignant lesion strongly predicted ifs malignant progression. Conclusion: Our results show that laryngeal tumor development involves chromosome tetraploidization. The further change from a stable to an unstable chromosome constitution is of importance for malignant progression. J Clin Oncol 18:1644-1651.