Radiotherapy(RT) is one of the most effective treatment modalities in head and neck cancers. However, the recent association of alterations in lymphocyte subpopulations and impaired cellular immunity prompted an investigation of the effects of RT on lymphocyte subsets in 28 patients with head and neck cancer. Percentage and absolute levels of CD3, CD4, CD8, CD16, and CD56 were respectively measured before RT, immediately after RT, 3 months, 6 months, and 112 months after RT. There were profound decrease in percentage and absolute level of CD3+ lymphocytes after RT, and these changes were still remained on 12th month after RT. The damage of CD4+ lymphocyte was more profound than CD8+ lymphocyte. Percentages of CD8+ lymphocyte and CD16+ and/or CD56+ cell(natural killer cell) were increased after RT, but absolute level of CD8+ lymphocyte was decreased and natural killer cell was not changed by RT. In conclusion, natural killer cell was most resistant to RT and CD4+ lymphocyte was most sensitive, and RT effects on peripheral blood lymphocytes were sustained for 12 months after RT.