Oral cancer has caused great concern in all of the western countries over the past two decades because of its progressively increasing incidence, mainly in young males, and its consistently low 5-year survival rate. Oral premalignant lesions (dysplasias) are thought to precede the development of cancer, but no clinical or histological criteria is at present available to predict their potential for malignant transformation. Therefore, it would be of diagnostic and therapeutic relevance to understand the molecular event, involved at different stages of oral cancer. Using a unique series of primary cultures from biopsies of normal oral mucosa, dysplasias and squamous cell carcinomas we have identified molecular changes characteristic of early oral cancer progression. Our group reported previously that acquisition of the immortal phenotype is an early event in oral cancer development (McGregor et al. 1997): data obtained during the course of this thesis project indicate that about half of oral dysplasia cultures are immortal and this is associated with loss of expression of retinoic acid receptor (RAR)-b and the cell cycle inhibitor, p16INK4a, p53 mutations and increased levels of telomerase/hTERT mRNA. In contrast, increased expression of the EGF receptor (EGF-R), known to be a characteristic of oral cancer, does not occur until after the dysplasia stage in squamous cell carcinomas (SCCs). Interestingly, one atypical mortal dysplasia with a considerably extended lifespan has lost expression of RAR-b and p16INK4a, but it still expresses wild-type p53 (albeit at higher level than normal) and has not activated telomerase. Further experiments demonstrate that retroviral transduction of hTERT immortalises D17 and this is associated with telomere lengthening but p53 remains wild-type. In contrast, transduction of hTERT does not extend the lifespan of two other typical mortal dyplasia cultures (that retain RAR-b and p16INK4a expression), even though telomeres are lengthened. Thus, telomerase activation/telomere maintenance is not sufficient per se to immortalise mortal dysplasias without concomitant loss of RAR-b and/or p16INK4a, but p53 mutation is not required if telomerase is activated exogenously. However, further work is required to clarify whether the molecular changes associated with the acquisition of an immortal phenotype in culture have any clinical or prognostic significance.