The tonsil is one of the mucosa-associated lymphoid tissue to protect the body from the entry of exogenous material through mucosal sites. Inflammatory diseases of the tonsil constitute the most commom cause of primary care visit to physicians and tonsillectomy represents one of the most common operations performed in children. However, the indications for tonsillectomy have been complicated by the controvery over the benefits of extirpating chronically inflammed tissues versus the possible harm which tonsillectomy may produce by eliminating an important local source of mucosal defense in the host. For determinig treatment options of tonsillar diseases, the authors performed an immunohistochemical staining for IgA immunocyte and measured the chages of serum levels of immunoglobulin A and secretory immunoglobulin A after tonsillectomy according to age in 126 cases(control 60 cases, tonsillectomy 66 cases). The results were as follows : 1) Levels of serum IgA between 11 and 13 years reached to adult's level in control. In tonsillectomy, levels of serum IgA below 10 years were higher than control and those were decreased to the levels of control after tonsillectomy of all ages. 2) Levels of salivary secretory IgA between 11 and 13 years reached to adult's level in control. In tonsillectomy, levels of salivary secretory IgA were decreased to the levels of control at postoperative 30th day, especially in 5 to 7 years and 8 to 10 years(p<0.05). 3) Histomorphometric features of tonsil above 8 years were more active than below 8 years in control. In tonsillectomy, histomorphometric features of those tonsil were similar with those above 8 years, but reticular part of crypt epithelium was remained significantly. 4) Total number of IgA immunocytes was the highest between 5 and 7 years being on the decrease with aging, especially in that above 19 years(p<0.05) in control. In tonsillectomy, the number of IgA immunocytes of extrafollicular area and reticular part of crypt epithelium was decreased below the level of control(p<0.05). In conclusion, tonsils of normal children under 13 years play a role in local immunologic defense mechanism, but the number of IgA immunocyte is decreased in tonsillar diseases despite of continuous exposure to exogenous antigens, perhaps as a result of inadequate stimulatory signals for local maturation of B cell clones. Decrement of serum and salivary secretory IgA to normal level after tonsillectomy may indicate that compensatory mechanisms are reflected in inversely related levels of local and systemic IgA synthesis. Therefore, tonsil in preschool age is important in local immunologic defense mechanism, but no significant changes in immunologic system occurs after tonsillectomy.