Completion thyroidectomy can most accurately be described as reexploration of the neck to remove the contralateral thyroid lobe. This procedure has commonly been performed when the histopathologic condition of the ipsilateral thyroid lobe reveals papillary or follicular carcinoma of the thyroid. Because of a definitely increased risk of complications with completion thyroidectomy, avoiding its routine use is important. The purpose of this paper is to define the specific indications for completion thyroidectomy.Over the past 9 years, we have performed 400 thyroidectomies; the patients ranged in age from 18 to 88 years. Although we have routinely used preoperative needle biopsy and intraoperative frozen section, decisions regarding the extent of thyroidectomy have been based on the gross findings at operation, taking into consideration such prognostic factors as patient age, tumor grade and size, the presence of extracapsular spread or distant metastasis, and associated risk factors. The minimal procedure for solitary thyroid nodule has been lobectomy with isthmusectomy. If the contralateral lobe is grossly normal, lobectomy with isthmusectomy has also been the maximal operation in most of patients. Very few specific indications exist for removal of the opposite lobe.Only three patients underwent completion thyroidectomy. All three patients had aggressive follicular carcinoma requiring radioactive iodine ablation, and all were referred after the initial surgery. We have not performed completion thyroidectomy on any of the patients on whom we initially operated.The most common indication considered for completion thyroidectomy is a frozen section diagnosis of a benign follicular adenoma that is subsequently changed on permanent pathologic condition to follicular carcinoma based on the presence of capsular and/or vascular invasion. However, if minimal invasion has occurred, no difference exists in survival related to the extent of the thyroidectomy. Local recurrence in the contralateral lobe occurs in less than 10% of the time. Because completion thyroidectomy carries more risks, it should be avoided in most patients when possible. The definitive decision should be made during the initial operation based on gross findings, prognostic factors, and frozen section, and this plan should only be changed to mandate completion thyroidectomy in select circumstances.