AbstractHuman telomeres consist of arrays of the sequence TTAGGG up to 15–20 kb in length, which are essential for the maintenance of normal chromosomal stability. It has been suggested that genomic instability observed in tumours may be due to loss of telomere sequences. Somatic cells that are dividing continuously appear to progressively lose telomere sequences, and it would therefore be anticipated that cell type specific differences in mean telomere length may exist within an individual. Previous reports have suggested that mean telomere length may be different in human neoplasia when compared to control. Basal cell carcinomas are epidermal derived tumours and in order therefore to make valid cell type specific comparisons we have measured mean telomere length in 20 basal cell carcinomas as well as in both adjacent epidermis and dermis. Mean telomere length was significantly reduced in epidermis in comparison with dermis, from clinically normal skin immediately adjacent to the tumours (mean difference 2.5 kb). This result is not related to the presence of the tumour as similar results were obtained from skin samples of healthy volunteers. Basal cell carcinomas showed increased mean telomere length in 13/20 samples in comparison with matched epidermis (mean difference 3.1 kb), whereas in 7/20 mean telomere length was reduced (mean difference 2.2 kb). These results showing that mean telomere length varies from cell type to cell type underpin the importance of performing cell type specific controls when assessing changes in tumour telomere