Despite ongoing developments of treatment protocols head and neck squamous cell carcinomas (HNSCC) show only marginal improvement in outcome, which has been attributed to a lack of therapy individualized to tumor biological properties. We compared mRNA expression profiles of HNSCC and normal epithelial cells using differential display to identify gene fragments showing differential expression in HNSCC cells. We identified a 127-bp long fragment to be overexpressed in HNSCC cells that revealed a 98.4% homology with the Pim-1 mRNA. The differential expression was confirmed by Northern hybridization. Immunohistochemistry showed overexpression of the Pim-1 protein in 98% (41/42) of invasive HNSCC. Analysis of Pim-1 protein expression in relation to TNM stage and histological grade of the tumors exhibited no significant correlation. However, when samples of primary tumor and metastasis retrieved from the same patients (n=26) were analyzed, nearly significant correlation of Pim-1 expression with histological grade was found (p=0.06). The high frequency of the Pim-1 expression of HNSCC of different grades and stages in conjunction with its absence in non-neoplastic head and neck squamous cell epithelium underlines the functional role of Pim-1 in molecular processes of HNSCC.