Serial measurements of lactate dehydrogenase (LDH) and of its isoenzymes have been assessed as an additional marker in a study of 44 patients with non seminomatous germ cell tumors of the testis. Its interest was compared with standard markers (AFP, beta HCG) related to clinical staging, histopathologic types and extent of the disease. Although LDH measurements were most frequently increased in stage III, its frequency of elevation was however lower than specific markers one. Abnormal levels of isoenzyme 1 could increased sensitivity of that determination. Indeed, LDH 1 was increased in all 15 cases for which it was determined, while total LDH was only disturbed in 8 cases. In patients with advanced testicular cancer and elevated LDH levels, sequential measurements of this parameter reflected the response to the therapy. A poor prognosis was associated with pathological level of LDH: while 92% of patients with normal level had a survival time upper than 5 years, no patient with elevated LDH had a survival time upper than 4 years; this difference was essentially significant in advanced stages. Serial measurement of LDH was of value as prognostic indicator and when elevated, it might be utilized as a guide for response to therapy.