We evaluated whether the serum soluble interleukin-2 receptor (sIL-2R) may be a parameter to monitor the efficacy of treatment for nasopharyngeal carcinoma (NPC). There were 177 NPC patients and 24 healthy controls. The level of sIL-2R was measured with a sandwich ELISA kit. Higher levels of sIL-2R than for controls were found in NPC patients before treatment and in patients with distant metastasis (p < 0.001). There was, however, no difference in sIL-2R levels between controls and NPC patients after radiotherapy in relapse-free or in primary relapse. The sIL-2R levels in sequential testing revealed good correlation with clinical response. The sIL-2R levels were found to be elevated when distant metastasis was detected. Two patients had elevated sIL-2R level up to 5 months before clinical detection of metastasis. These results indicate that serial measurements of sIL-2R levels are worthwhile for NPC patients in their clinical course. The sIL-2R level proved to be an adjunct clinical parameter to monitor the efficacy of treatment of NPC.