The quantitative distribution of the lactate dehydrogenase (LDH) isozymes is different and characteristic. Therefore, when the LDH isozymes are released from tissue to serum, as on cell injury, the serum LDH isozyme pattern changes and in a sense resembling the profile of the affected tissue. However, the serum LDH isozyme pattern often differs from the isozyme pattern in the original tissues because of differing individual isozyme elimination rates. On the contrary, we should be able to determine the origin of enzyme release, disease stage and prognosis. The LDH isozyme pattern is sometimes modified by various factors, such as LDH-immunoglobulin complex or genetic variants. Results of isozyme determinations must therefore be carefully interpreted with a knowledge of biochemistry, physiology and limitation.