A systematic evaluation of the anatomic relationships of the segmental bronchi, arteries, and veins of 107 right and 113 left lower lobes was made from CT scans of patients with normal chest radiographs. The classic taxonomies of Boyden and Jackson and Huber were used for nomenclature. Identification of individual structures was based primarily on careful analysis of contiguous CT slices. The frequency of identification of each of the major segmental bronchi and their corresponding arteries was established, and variations in the number and position of arteries were recorded at four transverse levels on the right side and three levels on the left side. The segmental venous tributaries of the inferior pulmonary vein were also identified. Although there is considerable constancy in the anatomy of the lower lobe segments, variation from the dominant pattern occurred in as many as 20% of cases. Sometimes major segmental bronchi were not identified and presumably varied in their origin. The segmental arteries may be single, duplicate, or even triplicate. Within the segments, the arteries generally lie toward the lung periphery relative to their corresponding bronchi, thus being anterior, lateral, or posterior. The segmental veins generally lie central to their bronchi, thus being posterior, medial, or anterior. Knowledge of prevailing patterns and variant appearances of the lower-lobe vascular structures can be helpful in interpreting CT scans. Such knowledge is a prerequisite for the identification of pulmonary nodules in the vicinity of vascular structures and the recognition of intersegmental lymph nodes and aberrant vessels.