The levels of natural killer cell activity(NK-CA) in the blood were determined in 117 patients with nasopharyngeal cancer and 40 healthy controls. The levels of NK-CA in the blood were significantly lower in patients than those in the control subjects(P<0.01). The NK-CA patients with moderately differentiated squamous cell cancer was significantly higher than that in patients with poorly differentiated squamous cell cancer and with undifferentiated cell cancer(P<0.05 and P<0.01,respectively). The mean levels of NK-CA in patients with TNM stages Ⅱ、Ⅲ、Ⅳand in those with recurrence were 25.75±14.63,24.70±12.42,19.15±8.29,15.65±7.25 respectively(P<0.05).A negative correlationship between the levels of EB-virus VCA-IgA antibody and the level of NK-CA in the blood was detected(P<0.05). The result has suggested that NK-CA in the blood in patients with nasopharyngeal cancer was depressed and thus lowered as the disease developed. The authors conclude that NK-CA in the blood might be regarded as one of the subsidiary factors for prognosis of nasopharyngeal cancer.