We measured serum lactate dehydrogenase levels in 36 patients with localized Ewing's sarcoma before treatment. The results were evaluated to determine if the levels served as a prognostic indicator for metastatic spread. After adjusting for primary site of disease and adjuvant chemotherapy, the pretreatment serum lactate dehydrogenase proved an extremely good predictor of which patients would ultimately develop metastatic disease. The median serum lactate dehydrogenase of the total group was between 201 and 214 IU/litre. Three of 18 patients presenting with serum levels of this enzyme of less than or equal to 201 IU/litre ultimately developed metastases, while 16 of 18 patients who presented with serum levels of greater than or equal to 214 IU/litre developed metastases (P less than 0.001).