Objectives: This study was undertaken: (1) to evaluate the usefulness of unenhanced computed tomography (CT), magnetic resonance imaging (MRI) and CT guided biopsy for the characterization of adrenal masses in patients with operable non-small-cell lung cancer (NSCLC) and (2) to evaluate the situations in which CT guided biopsy is absolutely necessary before potentially curative resection of NSCLC. Methods: Consecutive patients with operable NSCLC underwent unenhanced adrenal CT with density measurements of any adrenal mass over 1 cm in diameter. An adrenal mass was considered as an adenoma when its density was below 10 Hounsfield Units and a metastasis when its density exceeded 10 Hounsfield Units. Then patients underwent MRI, the signal on the T2 weighted images from the enlarged gland was classified adenoma or metastasis in comparison with that from the liver parenchyma. CT guided biopsy was performed after a pheochromocitoma was eliminated. Unenhanced CT attenuation values and signal intensity values on MRI were correlated with histopathologic results. Results: Of the 443 patients, 32 had an adrenal mass consisting of adrenal metastases in 18 cases and adenomas in 14 cases. On CT, 3/14 (21%) of the adenomas were misdiagnosed as metastases (their densities exceeded 10 Hounsfield Units) and 2/18 (11%) of the metastases were misdiagnosed as adenomas (their densities were below 10 Hounsfield Units). On MRI, none of the metastases were misdiagnosed as an adenoma (100% sensitivity) but 7/14 (50%) of the adenomas were misdiagnosed as metastases (signal superior to that of liver). Overall, a diagnostic certainty of metastasis could not be obtained in 25/32 patients (78%). CT guided biopsy with 100% sensitivity and specificity corrected all the inaccurate results of CT and MRI without any morbidity. Conclusion: Despite extensive morphological evaluation with unenhanced CT and conventional MRI, CT guided biopsy is necessary for most patients referred to surgery for an operable NSCLC and an adrenal mass.