Computed tomographic and pathologic correlations of the thymus gland were assessed in 69 patients. The sensitivity of computed tomography (CT) for undifferentiated thymic pathology is 87.1%; the specificity is 85.7%. The sensitivity of CT for neoplasm or mass is 97.1%, the specificity is 97.1%. The sensitivity of CT for lymphoid follicular hyperplasia (LFH) is 71.4%, the specificity is 97.6%. Therefore, a normal-sized thymus gland on CT does not exclude LFH. Completely preserved fat planes between thymic mass and adjacent structures on CT usually indicate a benign (noninvasive) neoplasm; completely absent fat planes usually indicate a malignant (invasive) neoplasm; partially preserved fat planes are indeterminate in assessing invasiveness. CT is also useful in showing recurrence or remnants of thymic tissue in patients who have had a previous thymectomy.