Eight women had painless thyroiditis, transient thyrotoxicosis, and low radioactive iodine uptakes but were without goiter; they constituted 15% of all thyrotoxicosis cases that we saw during the past year. Standard antithyroid antibody tests by tanned erythrocyte hemagglutination, complement fixation, and colloid and microsomal fluorescence, if present initally, were only weakly psotivie and became negative by 9 months. However, human antithyroglobulin antibody levels by a sensitive radioimmunoassay were elevated initially in all patients. In general, these radioimmunoassayable antibody levels fell, but they had not returned to normal by 9 months. Serial thyroid function tests and standard antithyroid antibody tests were most compatible with subactue thyroidtis. However, the persistence of radioimmunoassayable antithyroblobulin antibodies and recent reports of histologic evidence of lymphocyte thyroiditis in similar patients with goiter leaves open the possibility that this is a previously unrecognized presentation of chronic lymphocytic thyroiditis. Neither the cause nor the ultimate course of this syndrome is known.