INTRODUCTION: Selection of pulmonary staging modality in colorectal cancer surgery is controversial. Computed tomography (CT) clearly outperforms x-ray in terms of sensitivity, but findings of indeterminate lung lesions remain a problem. The aim of the present study was to evaluate the significance of such indeterminate lung findings in staging CT scans. MATERIAL AND METHODS: The study comprises a retrospective analysis of 131 consecutive patients who underwent colorectal cancer surgery in 2004. A preoperative staging CT scan of the chest and abdomen was performed in all patients. Twenty-six patients (20%) had indeterminate lung findings. Four died postoperatively. The remaining 22 were followed for a median period of 26 months. RESULTS: In eight of the 22 patients (36%) lesions progressed. In one patient, the lesion turned out to be a primary lung cancer, in another a lymphoma. In the last six patients (27%), the lesions developed into colorectal cancer lung metastases within a median period of 15 months. These results were significantly different from those obtained in patients who had a normal CT, among whom only 6% developed lung malignancies in the follow-up period (p < 0.0001). The development of lung metastases was significantly related to positive nodal status at operation and elevated carcinoembryonic antigen (CEA) level at follow-up (p < 0.05). CONCLUSION: Approximately one quarter of the indeterminate lung lesions found on staging CT in colorectal cancer patients developed into metastases and one tenth into other lung malignancies, which were most often diagnosed in the second year after surgery. The development of lung metastases was significantly related to positive nodal disease and postoperative CEA elevation.