To understand the biological events underlying the multistep process of oral tumorigenesis we have studied the expression of 70-kDa heat-shock protein, HSP70, in human normal, pre-malignant and malignant oral tissues. Expression of HSP70 was assessed in oral-tissue specimens using a mouse monoclonal antibody against HSP70 by immunostaining and immunoblotting analysis. Strong nuclear and cytoplasmic HSP70 immunostaining was observed in oral squamous-cell carcinomas (30/38). Mild to moderate HSP70 expression was observed in oral dysplastic lesions (19/30) and basal low level of HSP70 was observed in normal oral tissues. The results were corroborated by immunoblotting analysis. The wide variation in HSP70 expression in normal, pre-malignant and malignant oral lesions suggests that it is differentially expressed during oral carcinogenesis and may be implicated in tumor development.