Changes in the bronchial musculature in patients with chronic obstructive lung disease, were submitted to a systematic electron-microscopic investigation. A marked interstitial fibrosis with degenerative changes affecting the muscle cells was observed. The formal pathogenetic course of these changes can be reconstructed on the basis of numerous individual findings. The muscle cells are progressively "ensheathed" in a disorderly fine-filamentous basic structure (matrix), and in collagenous fibres. As a result, the contact surfaces between the muscle cells are disturbed, and the nerve endings "displaced" from the muscle cells. This fibrosis is associated with a protracted "shrivelling" of the individual muscle cells. The changes involving the muscles of the bronchial wall are comparable with those seen in the smooth muscles of blood vessels involved in arteriosclerosis. The morphological changes to the muscles of the bronchi described in this paper strongly suggest that protracted fibrosis formation, coupled with the degradation of muscle cells, contribute to a loss of elasticity of the bronchial wall in chronic obstructive lung disease.