Serum beta 2-microglobulin concentrations were determined in 21 untreated hyperthyroid patients (12 with Graves' disease, and nine with toxic nodular adenoma) and in 20 healthy controls. All subjects had normal serum creatinine concentrations and urine analysis. Both total and free thyroid hormones were significantly higher in the hyperthyroid groups than in controls. Beta 2-microglobulin concentrations were significantly increased in both groups of hyperthyroid patients compared with controls. No difference was found in the thyroid hormone and beta 2-microglobulin concentrations between both sets of patients. The beta 2-microglobulin and thyroid hormone concentrations were not correlated. These data show that hyperthyroidism is another cause of increased beta 2-microglobulin production along with viral infections, immunologically mediated diseases, and malignant neoplasms. The increased serum beta 2-microglobulin concentration in thyroid hyperfunction is probably related to metabolic rate, even if autoimmunity might contribute to its overproduction.