The decrease of immunoglobulin level after tonsillectomy was reported in literatures. The author observed the change of percentage of T lymphocyte before and after tonsillectomy in 21 patients, and found the percentage of T lymphocyte decreased from 60.0±5.2 preoperatively to 54.2±6.6 one month later postoperatively. This data suggested that cellular immunity was decreased transiently after tonsillectomy, so follow checkup was required. The immunological function of tonsil and change of immunity after tonsillectomy were reviewed.