Neoplastic diseases are currently one of the most common cause of mortality in both animals and humans. Therefore many of the recent research focuses on searching for biomarkers to enable qualitative and temporal improvement in the detectability as well as prognostic and predictive assessments in patients suffering from neoplastic diseases. One of such biomarkers seems to be cyclooxygenase-2 (COX-2) also known as prostaglandin G/H synthase (PTGS). The wide dissemination of COX isoforms in the tissues and of prostanoids synthesised with their involvement indicates the significant role they serve in the regulation of the functioning of several organs and systems, particularly the gastrointestinal, nervous, circulatory, reproductive, urinary and respiratory systems. Until recently, COX-2 was only associated with the response to stress and inflammation initiators. It is currently known that its expression increases significantly in the course of numerous pathological processes that involve inflammation, pain or fever. Results obtained during previous studies demonstrated a significant increase in COX-2 expression in several types and subtypes of neoplasms in humans and animals, particularly in dogs and cats, which have been studied the most frequently. Since COX-2 has not yet been introduced as one of the biomarkers for routine oncological diagnostics, this indicates the possibility for wider use of the assessment of its expression in the diagnostics and therapy of neoplasms in humans and animals. This suggest the possibility for using an assessment of COX-2 overexpression to improve cancer diagnostics and therapy and for prognostic and/or predictive purposes.