Blood lactate dehydrogenase is an aid in the diagnosis of diseases of the liver, certain tumors, and the heart. Lactate dehydrogenase is often markedly or moderately increased in acute hepatitis or active chronic hepatitis, and its clinical significance is roughly in line with that of alanine aminotransferase and glutamic oxalate aminotransferase. In hepatoma tissues, serum lactate dehydrogenase is markedly elevated in patients with hepatocellular carcinoma because the rate of glycolysis is significantly higher than in normal tissues. Therefore, if lactate dehydrogenase is found to be significantly elevated in patients with cirrhosis, the complication of hepatocellular carcinoma should be highly suspected. Serum lactate dehydrogenase is not specific for liver disease, but can also be elevated in myocardial, renal, blood cell and intestinal tissue injury. For example, myocardial infarction, pulmonary infarction, myositis, hemolytic anemia, pernicious anemia, and malignant tumors other than liver are common in clinical practice, all of which can lead to elevated lactate dehydrogenase. A comprehensive clinical analysis is needed to determine which aspect of the disease is involved.