Appendicular osteosarcoma (OSA) is a highly metastatic tumour in dogs. The aim of the study was to compare thoracic radiographs with thoracic computed tomography (CT) in the staging of canine appendicular OSA. In all, 39 canine patients histologically diagnosed with OSA were reviewed in the retrospective study. All dogs underwent radiographic examination as well as CT examination of the thoracic cavity. Pulmonary nodules were detected radiographically in two cases (5%), whereas the CT imaging showed that pulmonary nodules were evident in 11 cases (28%, P = 0.024). There was an improved detection of small pulmonary nodules in the lung parenchyma with CT (P = 0.021). The number of nodules in CT examination had a significant negative influence on survival time (P = 0.005). However, whether nodules were present in CT or not did not influence overall survival (P = 0.368). CT examination was superior to thoracic radiography in the screening and detection of pulmonary nodules in dogs with OSA.