It is well known and appreciated that the lungs contain organized lymphoid tissue. A few years ago while investigating the effect of the introduction of immune complexes into rabbit lungs, we observed that there were present in the bronchial walls lymphoid aggregates which resembled those in the intestine.l–3 Because the Peyer’s patches had been loosely included in the term gut-associated lymphoid tissue (GALT) we termed this bronchus-associated lymphoid tissue or BALT. On becoming much more interested in the nature of this tissue, and upon doing a literature survey, we found that this lymphoid tissue had been well described some 100 years previously but for obscure reasons in more recent years reference to it was infrequent. Klein,4 more than 100 years ago, observed such aggregates in the bronchial tract and noticed their similarity to Peyer’s patches. Similar lymphoid aggregates have been observed in primitive mammals by Miller5 and have now been identified in humans, rabbits, guinea pigs, dogs, rats, mice, swine and chickens.1–15