Among 2175 patients seen over the last three years in a non-specialized department of internal medicine with no intensive care unit, 100 had supranormal serum lactic dehydrogenase activities. These patients' case-reports have been analyzed. Nearly half the patients (47/100) had a malignant disease (cancer or hemopathy). Among the remaining patients, 19 had a hepatic disorder (alcohol hepatitis in 10, viral hepatitis in 8, and isoniazide hepatitis in 1), 7 had a heart disease (heart failure with hepatomegaly in 5, myocardial infarction in 2), and 27 had various other conditions (including hemolysis in 6 and polymyositis en 3). The value of serum LDH assay is obvious in situations other than acute conditions such as myocardial infarction of pulmonary embolism; these are better known and have not been studied here as their prevalence was low among the patients enlisted in our study. In comparison to other enzymes (alkaline phosphatase (AP), gamma-glutamyl transpeptidase (GGT), transaminases (GOT, GPT) that were also routinely assayed in our patients, abnormal serum LDH activities are much less common and their significance is quite different. An increase in serum and their significance is quite different. An increase in serum LDH activity indicates a serious condition, often with a fatal outcome. The "various other conditions" group includes patients with hemolysis, hepatitis and myositis; the other patients in this group either had severe infectious diseases or died suddenly in the first few days of their hospitalization before diagnosis had been established. Each etiologic group has been analyzed to asses the characteristics of patients with increased LDH activity according to each etiology. Analysis of coincident abnormalities of the other enzymes listed above shows marked differences between etiologic groups; diagnostic accuracy can thus be enhanced in certain conditions. Most patients with malignancies had poorly differentiated tumors, with metastases: 28 had an epithelial tumor, with hepatic and/or bone metastases in 23 cases, 5 had cancer of the liver, 10 had a malignant hemopathy (2 lymphomas, 5 myeloproliferative syndromes, 3 acute leukemias), and 4 had a sarcoma. Cancer of the lung is the most common malignancy (10 cases) and may be responsible for increased serum LDH activity even in patients without metastases. Serum LDH assay is of value for monitoring the course in patients with initially increased activities as it falls under effective therapy and rises during exacerbations.