Previous studies have suggested aberrant expression of membrane B7-H3 in tumor cells. This aim of the study was to determine the expression level of soluble B7-H3 (sB7-H3) in circulation and to subsequently evaluate the clinical significance of circulating B7-H3 in patients with non-small cell lung cancer (NSCLC). The level of circulating B7-H3 was determined with ELISA and its correlation with the clinical data was examined. Receiver operating characteristic (ROC) curve analysis was performed to compare the sensitivity and specificity in the diagnosis of NSCLC. Circulating B7-H3 levels in patients with NSCLC were significantly higher than those in patients with other pulmonary diseases (OPD, p<0.001), or those in healthy volunteers (p<0.001). Using a cutoff of 30ng/ml, the sensitivity and specificity of sB7-H3 in differentiating between patients with NSCLC and patients with OPD, and between patients with NSCLC and healthy volunteers was, 48.8 and 98.5%, and 48.0 and 93.7%, respectively. Additionally, higher levels of sB7-H3 were associated with higher tumor stage, tumor size, nodal metastasis, and distant metastasis, but not with sex, age or histological subtype. An area under the curve (AUC) for all stages of NSCLC resulting from sB7-H3 (0.862), which was significantly better than any other tumor markers tested including CA125 (0.621), CA153 (0.571), CA199 (0.459), and CEA (0.638). These results suggest circulating B7-H3 is a valuable biomarker for NSCLC and an elevated level of circulating B7-H3 suggests a poor clinical character for NSCLC.