We investigated the sub-clinical lung injury in patients with breast cancer after chemotherapy by technetium-99m hexamethylpropylene amine oxime (Tc-99m HMPAO) lung scan. Fifty patients who underivent surgical resection for breast cancers were divided into two groups, those who received a combination of adjuvant chemotherapy and those who did not. Group 1 consisted of 25 patients who received adjuvant chemotherapy and group 2 consisted of 25 patients who did not receive adjuvant chemotherapy. Tc-99m HMPAO lung scan was performed in each patient and the degree of pulmonary vascular endothelium damage was represented as lung/liver uptake ratios (L/L ratio). None of the 50 patients had positive plain chest radiograph (CXR) and pulmonary function test (PFT) findings. The results showed a significantly higher L/L ratio in the group 1 patients (0.57 +/- 0.13) than in the group 2 patients (0.32 +/- 0.05). We conclude that chemotherapeutic drugs may result in pulmonary vascular endothelium damage and that Tc-99m HMPAO lung scan has the potential to be a sensitive, objective and non-invasive method to detect sub-clinical lung injury of patients with breast cancers after chemotherapy.