Of 771 young diabetic patients, thyroid microsomal autoantibodies occurred in 136 (17.6%) at a female/male ratio of nearly 2:1 and with a predominance of white patients (20.1%) over black patients (5.5%) (P less than 0.001). Thus, one in every four white female patients with insulin-dependent diabetes mellitus had TMA. Thyroglobulin autoantibodies were no more common in patients with IDDM than among controls. Of the 117 patients (out of the 136) with serologic evidence of chronic thyroiditis who could be studied, eight (7%) had hyperthyroidism and 45 (38%) were hypothyroid. Hyperthyroidism usually preceded or coincided with the appearance of IDDM, whereas hypothyroidism occurred with or following the onset of IDDM. Hypothyroidism appeared irreversible in most patients, but in three, periods of hypothyroidism were followed by euthyroidism, presumably explained by a compensatory hyperplasia of the thyroid gland. In the 136 patients with TMA, gastric and adrenocortical autoantibodies also occurred at relatively high frequencies (16.8% and 5.1%, respectively). On the basis of these studies, we urge that all patients with IDDM be screened for TMA and that those with positive results undergo annual thyroid function tests as well as determinations of gastric parietal and adrenocortical autoantibodies.