The authors performed a retrospective analysis of serum soluble interleukin-2 receptor (sIL-2R) levels in 72 patients with nasopharyngeal carcinoma (NPC) using an enzyme immunoassay. Their objectives were to determine the value of serum sIL-2R in estimating the tumor burden, and its predictive value in response to therapy and prognosis. The data showed that serum sIL-2R levels in patients were significantly higher than that of healthy controls. The serum levels correlated with clinical staging and hence the tumor burden of NPC. Serial measurement of serum sIL-2R provided an accurate prognostic index of the clinical response to radiotherapy in at least 89% of patients with raised serum sIL-2R at initial diagnosis (defined as mean + 2 SD of healthy controls) and a reliable predictive index in all patients who subsequently developed distant metastasis despite initial radiotherapy. Simultaneous measurement of Epstein-Barr virus-related serology (IgA-VCA and IgG-EA) failed to demonstrate predictive value comparable with that of serum sIL-2R. The authors conclude that monitoring serum sIL-2R levels has clinical and prognostic significance in patients with NPC and that prospective studies are indicated.