Background: The purpose of this study was to evaluate the value of circulating non-hematologic cells to differentiate benign from malignant lung lesions and their comparison with clinico-histologic features of corresponding primary lesions. Methods: Circulating cells were isolated by size method from peripheral blood of 77 patients with malignant (n = 60) and benign (n = 17) lung lesions. They were morphologically classified as cells with malignant feature; cells with uncertain malignant feature; and cells with benign feature; then statistically correlated with clinico-cytopathologic characteristics of corresponding lung lesion. Results: Malignant circulating cells were detected in 54 of 60 (90%) malignant patients, and in 1 of 17 (5%) benign patients; benign circulating cells in 1 of 60 (1%) malignant patients and in 15 of 17 (88%) benign patients; and circulating cells with uncertain malignant aspect in 5 of 60 (8%) malignant patients and 1 of 17 (5%) benign patients. For a malignant circulating cells count greater than 25, sensitivity and specificity were 89% and 100%, respectively. The count was significantly correlated with stage, size, and standard uptake value of primary tumor. In 39 of 54 (72%) cases, the malignant circulating cells allowed a specific histologic diagnosis of the corresponding primary tumor after immunohistochemical analysis. Conclusions: Malignant circulating cells may be a valid marker in the diagnostic workup of lung lesions. However, our resuts should be corroborated by larger future studies especially for patients having small nodules.