The size of the remnant gland after subtotal thyroidectomy varied between 5 and 12 g in 85 hyperthyroid patients. The patients were then examined 6 weeks and 6 months after surgery with estimation of TSH and TRH stimulation test as well as thyroxine and triiodothyronine. A significant correlation between TSH response to TRH and remnant weight was found ( r= 0.94, p less than 0.01). The frequency of raised S-TSH was higher in the group with small remnants compared with those with larger ones. There was only minor correlation between raised S-TSH or high TSH response to TRH and remnant size expressed in per cent of the total thyroid weight (relative remnant size). No significant difference in thyroxine and triiodothyronine was found between the weight groups. Hypothyroidism developed in two patients with small remnants (6 g) and in one patient with a remnant weight of 8 g. Recurrent hyperthyroidism occurred in one patient in the 8-gram weight group. These results indicate that the remnant size may be one factor influencing the outcome of thyroid function after surgery.