The CT density of the same pulmonary nodule can vary significantly between scanners or with the same scanner because several independent factors besides partial volume averaging can affect its determination. Hence a single CT number cannot be used to distinguish calcified from noncalcified nodules, ruling out direct extrapolation of quantitative data between scanners. The authors designed a phantom that simulates CT measurements in patients and permits comparison of CT density of each nodule with a physical standard derived from clinical experience. Tests on 35 patients using a GE 8800 showed that no malignant nodules and 65% of benign lesions were more dense than the phantom nodule. This method is independent of inter- and intra-scanner variation and facilitates standardized quantitative analysis of pulmonary nodules with current scanners.