An analysis was made of chromosomal abnormalities in surgical specimens of human thyroid glands. Three histologically proved normal control thyroid glands were obtained at laryngectomy. Eight histologically proved colloid-nodular goiters, 5 goiters of Graves'' disease, 3 goiters of Hashimoto''s struma, 2 carcinomas were studied. The thyroid glands from patients with each of the 5 thyroid disease categories were demonstrated to have a greater percentage of aneuploid cells than the normal mean of 1. 7% [plus or minus] SE of 1.20. The thyroid tissue from each disease state had a higher degree of tetraploidy and hypertetraploidy than normal controls. Graves'' disease, colloid nodular goiter and Hashimoto''s specimens showed a significantly greater degree of tetraploidy and octaploidy than adenoma and carcinoma specimens. Chromosomes were missing only in the colloid-nodular group. Additional chromosomes were seen in colloid-nodular goiter and in carcinoma. Marker chromosomes were found only in Graves, disease (2 patients). Although these chromosomal abnormalities are difficult to interpret in terms of a cause and effect relationship, the findings are of interest in relation to the sequential morphological and chromosomal studies we have previously re ported in the Fischer rat maintained on a low iodine diet. As the thyroid glands of these rats developed hyperplasia under the stimulus of a low iodine diet, an Increasing percentage of imperfect divisions of chromosomes was ob-served. Unusual hyperplasia was a common finding in each of the human thyroid diseases studied here.