Background: Telomerase activity is required for both initiation and maintenance of tumorigenesis and over 90% cancers overexpress telomerase. Therefore, telomerase targeting has emerged as a potential strategy for cancer treatment. In agreement with this, several telomerase inhibitors are being tested for cancer treatment and have shown some promise. However, because of the variability in response between the cancer patients, it is important to identify biomarkers that allow for distinguishing cancers that are responsive to telomerase inhibition from the cancers that are not. Therefore, in this study we performed experiments to identify a biomarker that can be used to predict telomerase inhibition induced tumor growth inhibition. Methods: In our study, we have performed transcriptome-wide gene expression analysis on multiple ovarian and colon cancer cell lines that were treated with telomerase inhibitor imetelstat and were responsive to telomerase inhibition-induced tumor growth attenuation. Results: We demonstrate that telomerase inhibition by telomerase inhibitor imetelstat results in decreased expression of interleukin 8 (IL8) in all telomerase responsive cancer cell lines. This phenomenon is of general occurrence because we find that multiple ovarian and colon cell lines show decrease in IL8 mRNA and protein levels after telomerase inhibition. Additionally, we find loss of IL8 phenocopy Telomerase inhibition mediated growth inhibitory effect in cancer cells. Conclusion: Taken together, our results show that IL8 is a biomarker that predict telomerase inhibition mediated growth attenuation of cancer cells and its loss phenocopy telomerase inhibition. Therefore, IL8 expression can be utilized as a biomarker for telomerase targeted cancer therapies to potentially predict therapeutic response.