Hitherto voice changes have been regarded as an infrequent complication of thyroidectomy and damage to the recurrent laryngeal nerve has been given as their major cause. Voice function was assessed in 325 patients after thyroidectomy. Permanent changes occurred in 35 (25%) after subtotal thyroidectomy and in 19 (11%) after lobectomy. The commonest cause of voice change appeared to be injury to the external laryngeal nerves on one or both sides. Damage to the recurrent laryngeal nerve, which was routinely identified and protected, was rarely a cause. When the external laryngeal nerves were identified and preserved, permanent voice changes occurred in only 5% of cases; this was similar to the incidence of 3% in controls after endotracheal intubation alone. The course of the external laryngeal nerve is variable, and consequently mass ligation of the vessels at the top of the upper pole will damage it in a high proportion of cases. To minimise this serious complication these nerves should be identified and protected as well as the recurrent nerves and voice function should be assessed early in the postoperative period by laryngoscopy and by a speech therapist.