Four women and one man with painless subacute thyroiditis presented with hypermetabolic signs and symptoms. Thyroxine (T4) and triiodothyronine (T3) resin uptakes (T3R) were increased but the 24 hour radioactive iodine (RAI) uptakes were less than 1 per cent. Surreptitious use of thyroid hormone was excluded. The thyroid was enlarged in one patient and nontender in all. Exophthalmos was absent. The protein-bound iodine level was 1.1 to 9.5 mug/dl greater than the T4 level. The sedimentation rate was normal or minimally increased, and antithyroglobulin and antimicrosomal antibodies were undetectable. In one hospitalized patient 84 per cent of the administered dose of 131I was recovered in the urine within 48 hours (normal 64 per cent) excluding extrathyroidal uptake. In all subjects the T4 and T3R levels fell to normal or slightly below normal within one to four months. An increase in the 2 and 24 hour RAI uptake to minimally increased or high normal values and return of the T4 and T3R levels to normal occurred in four of five patients within six months. In one of these, the administration of thyroid-stimulating hormone (TSH) resulted in an appropriate increase in 24 hour RAI uptake from 14.9 to 37.1 per cent. One woman remained clinically hypothyroid for six months with a low T4 concentration (3.2 mug/dl), an elevated TSH level (48 muU/ml) and evidence of a persistent organification defect -- two hour RAI uptake decreasing from 33 to 23 per cent after the administration of perchlorate and the 24 hour RAI uptake increasing from 32 to 76 per cent following the administration of TSH. At 21 months after the initial onset of her illness, she is euthyroid but increased RAI uptake persists. The clinical course in four of the five patients is similar to that in an additional eight patients treated during the same time period who presented with typical subacute thyroiditis. Thus, these patients have a form of painless subacute thyroiditis which presents as thyrotoxicosis but is differentiated from it by a low RAI uptake and in whom recovery of thyroidal iodine trapping is the first indicator of recovery. The hyperthyroidism is self-limiting and should be treated conservatively.