The two main reactive pulmonary lymphoid disorders are lymphoid interstitial pneumonia and follicular bronchitis/bronchiolitis, both pathological entities with a variety of aetiologies. We reviewed the morphological and immunohistochemical features of 26 cases with one or other of these two diagnoses, to explore the possibility that they represented overlapping patterns of hyperplasia of the bronchopulmonary immune system. The polymerase chain reaction was used to determine the clonality of the infiltrates. Histologically, there was a spectrum of changes with two main components. An interstitial infiltrate of mainly T lymphocytes, plasma cells and histiocytes predominated in lymphoid interstitial pneumonia, whilst lymphoid follicles predominated around airways in follicular bronchitis/bronchiolitis. Classification of the disorder rested on which component the pathologists believed to be dominant. In two cases, histology and immunohistochemistry suggested lymphoma, and in one of these cases this diagnosis was confirmed by the polymerase chain reaction. One case of lymphoid interstitial pneumonia produced three bands. The remainder produced polyclonal patterns when samples were adequate. Clinically, there was no clear difference between patients with the two disorders, or patients with pathological features of both.