The development of constrictive bronchiolitis obliterans in patients who have ingested Sauropus androgynus juice has not been previously reported. We describe four patients with S. androgynus-associated constrictive bronchiolitis obliterans who underwent open lung biopsies for histopathological analysis. This article aims to recognize the possible pathogenesis of the disease. The spectrum of histologic changes ranged from slight bronchiolar inflammation and fibrosis to marked submucosal fibrosis causing complete cicatricial obliteration of the lumen. A dense eosinophil infiltrate was noted in the bronchiolar submucosa or fibrotic tissue of the completely obliterated bronchioles in two patients. Immunohistochemical studies revealed that lymphocytic infiltrate consisted mainly of T lymphocytes in all patients. Immunofluorescent stains for immunoglobulin (Ig)G, IgA, IgM, C1q, C3, and C4 were negative. Electron microscopy disclosed no immune complex deposition in the specimens examined. Although the role of antibody- and complement-mediated reaction is not supported by the negative results of immunofluorescent and electron microscopic studies, the common finding of the predominant T-lymphocytic infiltrate suggests that a T-cell mediated immune response is involved in the pathogenesis of the disease. Furthermore, the finding of a heavy lymphocytic infiltrate in many bronchioles without significant collagen deposition suggests that the lymphocytic infiltrate may precede the tissue fibrosis. In addition, the presence of a dense eosinophil infiltrate may combine with lymphocytes and other immunologic and mesenchymal cells to promote antigen-specific stimulation of lymphocytes and induction of fibrosis.