The exact architecture of the normal nasopharyngeal tonsil remains obscure because most histopathologic investigations have been based on surgically removed adenoids. We compared enlarged adenoids and normal nasopharyngeal tonsils under both light and electron microscopes. The marked features of clinically enlarged adenoids were a large extension of the reticular epithelium and increased germinal centers. A tendency toward increased stratified squamous epithelium and decreased ciliated epithelium was apparent in enlarged adenoids, possibly due to inflammatory conditions. One type of nonciliated cell seemed to transport foreign material into underlying lymphocytes, as do the M cells of gut-associated lymphoid tissue. This type of nonciliated cell was rarely found in the extended reticular epithelium of enlarged adenoids. These findings suggest a disturbance of the antigen-trapping system and surface protections in adenoidal enlargement.