The pretreatment serum lactic dehydrogenase (SLDH) levels of 618 patients with Ewing's sarcoma of the extremities (136 metastatic at presentation and 482 localized) were analyzed to evaluate whether the enzyme level had a clinical value in predicting the course of the disease. The percentage of patients with increased SLDH was significantly higher in the metastatic group than in the group of patients with localized disease (68% vs 32%; P<0.0001). In the latter group treated with neoadjuvant chemotherapy the 5-year disease-free survival rate was significantly higher in patients with normal pretreatment SLDH than in those with high levels (65% vs 41%; P<0.0001). The time to relapse was significantly shorter for patients with elevated SLDH than in patients with normal values of the enzyme. The site of the tumor was significantly related with the stage of the disease, and for patients with localized disease, with the disease survival rate, at the multivariate analyses site of the tumor and SLDH levels were independently related with the stage of disease and with prognosis. These data demonstrate that in Ewing's sarcoma of bone pretreatment SLDH have a prognostic value and should be considered in the comparison of the results achieved with different therapies and in planning new randomized clinical therapeutic trials.