The serum lactase dehydrogenase (LDH) of 56 patients with Ewing's sarcoma was examined to determine the use of LDH as a tumor marker. The initial LDH level was used to determine its ability to predict survival. The follow-up LDH level also was examined, as an indicator of recurrent disease. The initial LDH level was found to have no prognostic value. Both patients with elevated and normal LDH levels had the same survival rate. The LDH level at recurrence was significantly higher, however, than the follow-up LDH level of those without recurrence (P less than 0.001). The LDH level was most sensitive as a tumor marker for recurrent disease in those patients with multiple sites of tumor involvement at the time of recurrence.