The frequency of smoking-induced nasopharyngeal lymphoid hyperplasia in heavy smokers and its potential clinical implications are still unknown. Precise criteria to differentiate this entity from other types of nasopharyngeal lymphoid hyperplasia are needed. A prospective clinicopathological study of smoking-induced nasopharyngeal lymphoid hyperplasia was conducted in 17 heavy smokers. Ten nonsmoking patients, five of them with chronic sinusitis, three with adult-onset adenoid hypertrophy, and two children with adenoidal hypertrophy served as a control group. Both in smokers and in nonsmokers, lymphocytic infiltration of the mucosa was characterized immunohisto-chemically as T cells. In smokers, semithin (1 micron) sections revealed deformed and migrating cytotoxic lymphocytes in the nasopharyngeal mucosa. The lymphocytes were attached to epithelial, ciliated, and goblet cells, resulting in cell damage. Transmission electron microscopy of biopsies from smokers revealed emperipolesis, characterized by mucosal invasion and epithelial cell damage by an unusual population of migrating T lymphocytes that penetrate them. These findings confirm a direct effect of smoking on the nasopharyngeal lymphoid tissue, which forms part of the immune system. It is concluded that the diagnostic evaluation and therapeutic approach of heavy smokers with otological and airway symptoms should be based on thorough endoscopic examination of the nasopharynx. When the diagnosis is not clear-cut, selective tele-endoscopic biopsy and electron microscopic examination are recommended. This entity should be added to the list of known clinical manifestations of the smoking habit.