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Since second half of the 20th century lung cancer occurrence has increased rapidly and nowadays lung cancer accounts for one million and six hundred thousands new cancer cases and nearly one million five hundred thousands deaths worldwide per year.Lung cancer is the main cause of deaths related to malignant diseases.Because of nearly asymptomatic onset of the disease, early diagnostics of lung cancer is highly unsatisfactory and only 1 0-15% of patients survives 5 years.Moreover, a high percentage of lung tumours is located peripherally in the lung parenchyma and a lot of suspected peripheral lung infiltrations are finally benign lesions.These conditions make histological diagnosis of lung cancer more difficult, so new diagnostic tools have to be looked for.According to Hanahan and Weinberg,there exist six fundamental routes of carcinogenesis associated with activation of different groups of genes and signals.Those acquired capabilities of mutating cells are: self-sufficiency of cell in growth signals, insensitivity to anti-growth signals, lack of apoptosis, not limited replication, promotion of angiogenesis and tissue invasion.According to current knowledge regulation of cell replicative capability first of all depends on telomeres length and telomerase activity.Telomerase, an enzyme associated with cellular immortality, is highly expressed in the majority of cancers.Assessment of telomerase activity could be helpful in differentiation between benign and malignant peripheral tumours of lungs and could improve sensitivity of cytological examination of poor-cell aspirates suggesting malignant aetiology of the suspected lung lesions.High telomerase activity in tumour cells is an unfavourable independent prognostic factor of survival.It seems that telomerase is also promising target for anti-cancer molecular therapy.This presentation reviews current knowledge about importance of telomerase in carcinogenesis and its clinical usefulness.