Sixty-seven patients with thyroid disease and 24 normal controls were examined with computed tomography (CT). The mean CT number (Hounsfield units +/- SD) in the normal controls (118.1 +/- 12.2) was significantly higher (p less than 0.001) than the mean CT number in patients with diseased thyroids, except for 2 cases of simple goiter (CT numbers 113, 132). The mean CT numbers in the patients with Graves disease (69.5 +/- 17.6) and Hashimoto thyroiditis (61.4 +/- 9.1) were significantly higher than those in patients with adenoma (41.7 +/- 10.6, p less than 0.001), cyst (33.1 +/- 14.8, p less than 0.001), or cancer (48.7 +/- 13.0, p less than 0.01). In 14 patients studied, a significant correlation was observed between thyroid CT numbers and the iodine concentration of the tissue (r = 0.889; p less than 0.001).