In 100 right human lungs the main bronchus, the pulmonary artery and the pulmonary vein were injected with 65% methyl methacrylate and then digested in sulphuric acid. The resulting specimens were studied to observe the divisions of the middle lobe bronchus and the types of arterial and venous vascularization of this lobe. The lobe was always entered by one lobar bronchus, which usually divided into two segmental bronchi. In 53% of the middle lobes with this bronchial pattern there was one artery. When the lobar bronchus divided into three branches, at least two arteries entered the lobe in almost all cases. Complete consistency between the pattern of bronchial division and that of arterial vascularization of the middle lobe was found in almost two-thirds of cases. Associations between patterns of bronchial division and of venous drainage from the middle lobe were found in slightly more than half of the cases.