Bronchus-associated lymphoid tissue (BALT) has been defined as the organized lymphoid tissue of the lung. Although well described in a variety of animal species, documentation of its presence and development in human lung is limited. Because the tissue to volume ratio in adult lungs is so low, a systematic search for BALT would involve so many sections as to be impractical. In this study, therefore, we have studied post-mortem specimens of fetal (n = 102) and infant (n = 17) lungs, which have a much higher tissue to volume ratio. Fetal death was due to various causes but all but two infants died from sudden infant death syndrome. In the fetal lungs, the presence of BALT was almost invariably associated with chorioamnionitis or intrauterine pneumonia, being present in 24 of 51 of these cases (47 per cent). The earliest ill-defined lymphoid aggregate was seen at 16 weeks' gestation, while lymphoepithelium, a hallmark of mucosa-associated lymphoid tissue, could be identified at 20 weeks. In 51 fetuses without infection, BALT was found in only five cases (10 per cent). BALT was identified in 13/17 (77 per cent) of infant lungs and well-developed lymphoepithelium was evident in four cases. This study shows that BALT may be present in the human fetal and infant lung, but that its appearance is probably dependent on antigenic stimulation.