Observation of bronchus-associated lymphoid tissue (BALT) in whole lung specimens from healthy nonsmoking adults has questioned the hypothesis that BALT is not constitutively present in healthy adult human lungs. In our study, we investigated endobronchial biopsies of the second- and third-generation carinae from 44 cross-country ski athletes and 12 healthy control subjects, all nonsmoking young adults. The skiers had a prevalence of respiratory allergy (18%), asthma-like symptoms (59%), beta2 agonist medication (25%), and methacholine bronchial hyperresponsiveness (79%). Biopsy sections were stained by immunohistochemical and hematoxylin-eosin-saffran methods. Lymphoid aggregates of more than 50 cells were identified in 28 (64%) skiers and three (25%) control subjects (p = 0.02). They were small in comparison to those found in rabbits and rats, contained T and B lymphocytes and macrophages, and were seen more frequently in skiers using beta2 agonists (p = 0.04) and with bronchial hyperresponsiveness to methacholine (p = 0.053). The frequency of these aggregates was not significantly different at the two carinal levels (p = 0.6). The aggregates were not associated with a history of respiratory allergy or asthma-like symptoms. These aggregates share some resemblance with what is usually defined as BALT. However, their exact nature and function await further clarification.