Episodes of transient thyrotoxicosis after surgery for primary hyperparathyroidism have previously been described, and surgical trauma to the thyroid gland has been suggested as an etiologic factor. However, there are several links between the thyroid and parathyroid hormonal systems, and therefore other explanations are possible as well. In this study we investigate pre- and postoperative serum levels of thyroid hormones in 20 patients operated upon because of primary hyperparathyroidism. The mean (SD) serum levels of T4 increased from 16(2) pmol/l preoperatively to 21(6) pmol/l on the fourth postoperative day (P < 0.01), and four (20%) of the patients developed biochemical thyrotoxicosis in the immediate postoperative period. The serum levels of T4 on the fourth postoperative day correlated highly with preoperative serum levels of PTH (r = 0.75; P < 0.001). This suggests that biochemical thyrotoxicosis in the immediate postoperative period after operation for primary hyperparathyroidism is not uncommon and could be related to the disease rather than to surgical trauma.