The clinical stage of cancer is the end result in a complex, multistage carcinogenic process, during which several regulatory circuits are altered. These alterations often manifest themselves through changes in the cellular enzymatic patterns. It is well known that the most prominent biochemical feature of malignant cells remains the high glycolytic activity, the result of which is production of a large quantity of lactic acid. Lactic acid dehydrogenase (LDH) is considered a key enzyme in glycolysis. It catalyses the process of lactate production in pirogronian reduction reaction. Five active isoenzymes of LDH can be found in human tissues. Each isoenzyme is a tetrametric composition of two subunits, A(M) and B(H), encoded by separate genes (Ldh-A and Ldh-B respectively). Each subunit determines metabolic characteristics of isoenzymes and predisposes them to activity either in aerobic (B-type isoenzymes) or anaerobic environment (A-type isoenzymes). Therefore each normal tissue has its own special LDH activity pattern, depending on its function. In malignant tumours the macromolecular synthesis is changed in favour of subunit A in comparison with the healthy tissues from which they derive. It results in a dominance of the cathodic isoenzymes, which move slowest after electrophoresis, namely LD5 and LD4. It is even thought that izoenzymograms of most malignant tumours do not differ. Similar changes in isoenzyme activity of LDH are observed in morphologically normal tissues, adjacent to the tumour, which can suggest early pre-tumour stage in those tissues. In this report we summarise basic physico-chemical properties of LDH and discuss its clinical significance in diagnosis and prognosis of neoplastic diseases.