In 127 patients with squamous cell carcinoma of the upper aerodigestive system, an assessment of natural killer (NK) cell function was performed. The mean lytic unit (LU) value of this cancer population was noted to be less than the mean value of 67 age-matched controls assessed concurrently. The major determinant of cytolytic function was related to the growth pattern of the tumor. Increased NK cell function was observed in patients with lesions that were more locally or regionally aggressive, i.e., that infiltrated surrounding anatomic structures. The magnitude of NK cell response also correlated with increased amounts of circulating IgG immunoglobulin to herpes simplex virus-type 1-associated antigens; elevated IgG levels were also associated with locally aggressive lesions. The clinical significance of NK cell activity in these patients is shown by its relationship to disease-free prognosis. Patients with elevated NK activity followed for a mean of 12 months had an improved disease-free survival as compared to the survival of the remaining population. Furthermore, NK LU values were not reflected in standard staging methods, which suggests that the measurement of NK cell function represents an independent prognostic parameter in the patient with head and neck cancer.