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OBJECTIVE To better understand spatial relationships between principal bronchi and other intrathoracic structures by use of CT images of dogs of various somatotypes. ANIMALS 93 dogs that underwent thoracic CT. PROCEDURES Information was collected from medical records regarding signalment and physical examination and echocardiographic findings. Two investigators recorded multiple measurements on a thoracic axial CT image from each dog. RESULTS Thoracic height-to-width ratio (H:W) was associated with left principal bronchus (LPB) and right principal bronchus (RPB) H:W, aortic-LPB separation, focal LPB narrowing, and aortic-vertebral overlap. Thoracic H:W was not associated with dog age, weight, sex, or brachycephalic breed. Twenty-five (27%) dogs had focal LPB narrowing, compared with 5 (5%) dogs with focal RPB narrowing (P < 0.001). Ten of 25 dogs had overlap or contact between vertebrae, aorta, LPB, and heart, suggesting a cumulative compressive effect on the LPB, while 15 had LPB-aorta contact and lack of contact between the aorta and thoracic vertebrae, suggesting an aortic constrictive effect on the LPB. None had LPB narrowing without contact from surrounding structures. Inter-rater agreement was high. CLINICAL RELEVANCE In dogs that underwent CT and were not selected for clinical suspicion of bronchial disease, principal bronchial morphology was associated with thoracic conformation. Focal LPB narrowing occurred more often than RPB narrowing. Focal LPB narrowing occurred with evidence of extraluminal compression, with or without contact between aorta and vertebrae. Brachycephalic breed could not be used for predicting thoracic H:W.