Impaired cell-mediated immunity has been consistently demonstrated in patients with head and neck squamous cancer(HNSC). Twenty-eight patients with HNSC were studied to assess potential abnormalities in T₃ cell, T₃/T₄ ratios, and natural killer(NK) activity. The control group consisted of 295 age-matched healthy person. The male : female ratio was 1 : 2. Compared with the controls, the patients with cancer before treatment had a significant decrease in T₃ cells(p<0.01), with the greatest reduction occuring in groups of well-differentiated squamous cell carcinoma(p<0.001), and stage II disease(p<0.001), but substantial differences in T₄/T₈, ratio and NK activity were not significant. Also between the patients group before and after treatment, substantial differences in T₃, T₄/T₈ ratio, and NK activity were not detected. The conclusions of this study are : 1) The measurement of T-cell subpopulation and NK activity offer no implicacation of the development of the head and neck cancer, and the prognosis after treatment. 2) Only the reduction of T₃ cell population in peripheral blood may be related to the development of the head and neck cancer. 3) Longer follow-up and expansion of the data base will be necessary before a determination can be made of the value of quantitative T-cell subpopulation and NK activity analysis relative to their use, as prognostic indicators in patients with head and neck cancer.