We evaluated the findings on transbronchial biopsy specimens in reference to open lung biopsy specimens from 12 patients with pulmonary eosinophilic granuloma. Features in transbronchial biopsy specimens were further contrasted to those of patients with interstitial fibrosis and nondiagnostic biopsy specimens for localized lesions. Transbronchial biopsy specimens were randomized and graded for histologic features and cellularity. Patients with eosinophilic granuloma had more macrophages (P < .05) but equivalent numbers of eosinophils, neutrophils, and Langerhans' cells compared with those of the other two groups. Only two endoscopic biopsy specimens were histologically diagnostic or highly consistent with eosinophilic granuloma. We conclude that the diagnosis of eosinophilic granuloma is possible on transbronchial biopsy but requires a high index of suspicion. The demonstration of Langerhans' cells by immunohistochemical staining for S100 protein is a useful adjunct. The low diagnostic yield for eosinophilic granuloma on transbronchial biopsy results from inadequate sampling and from the nonspecific appearance of discrete lesions in small tissue samples.