OBJECTIVE To determine the possibility of definitive diagnosis for solitary pulmonary nodules in patients with a primary extrathoracic malignant neoplasm (ETM-SPN), and to further evaluate the value of CT for differential diagnosis in ETM-SPN by a multivariate retrospective study.METHODS Eighty-three patients with pathologically and clinically proven ETM-SPN with a diameter smaller than 3 cm were included in this study.The pathological characteristics of the SPN were correlated with those of the extrathoracic neoplasm, with the patients age, gender, smoking history, disease-free time interval between the diagnosis of the extrathoracic malignancy and that of the lung lesion. In all 83 cases, CT scans were reviewed to confirm the solitary nature, size, and nodular morphology of the lung lesion.RESULTS Of all 83 cases, the mean age was (57.43±15.34) years. There were 51 males and 32 females, with the ratio of 1.59:1. The lesions included solitary metastasis in 43 cases, pulmonary malignant lesions in 33, and benign lesions in seven. Between the primary lung cancers and solitary metastasis groups, there was no significant difference in the gender ratio (1.20:1 vs 2.31:1, x2=0.0209, P>0.05), but there was a significant difference between the mean age (62.48±11.96 years vs 54.10±16.49 years, t=3.34, P0.05). Of 81cases who were followed-up, the mean time of the disease-free interval between extrapulmonary malignancy diagnosis and pulmonary lesion differentiation was 39.73± 6.29 months (range 0～300 months, median 20.00 months), whereas those in the primary lung cancer group and metastatic group were 65.62 ±13.45 months and 22.83 ±4.19 months respectively. This difference was significant between the two groups (Wilcoxon rank sum test, U=2.796, P0.05), without showing a statistically significant relevance between the pathologic patterns of extrapulmonary malignancy and characteristics of the lung nodules. Of all the 83 cases, the mean diameters were (2.77±1.25) cm, whereas the diameters of 33 cases of primary lung cancer and 43 cases of a solitary metastatic lesion were (2.86±1.18) cm and (2.62±1.31)cm, respectively. There was no association between the two groups (t=1.29, P>0.05). There was a statistically significant association between primary lung cancer and the metastatic group with spiculate and smooth edges of the lung lesion (x2=8.562, P