The clinical and pathological data were reviewed on 17 patients with benign endobronchial mesenchymal tumors seen at the New England Deaconess Hospital between 1942 and 1981. The tumors were compared histologically to 147 intraparenchymal hamartomas studied during the same period. Thirteen tumors involved central bronchi and four occupied small, peripheral airways. These 17 lesions displayed overlapping histological features and ranged from predominantly cartilaginous to lipomatous growths. Mixtures of cartilage and other mesenchymal elements characterized 14 tumors, and adipose tissue was the principal component of three nonchondromatous growths. Both endobronchial and parenchymal tumors occurred predominantly during the fifth and sixth decades. Tumors of both groups contained similar mesenchymal constituents; however, the endobronchial lesions had fewer epithelial clefts, a decreased proportion of cartilage, and a correspondingly increased adipose tissue component. The centrally located intralumenal growths caused symptoms due to bronchial obstruction, while the four tumors within peripheral airways were usually symptomless, incidental findings. The histological similarity between the to groups and the presence of some endobronchial tumors within small, peripheral airways support the theory that endobronchial and parenchymal "hamartomas" are similar mesenchymal neoplasms of bronchial and bronchiolar walls occurring at different loci within the bronchial tree.