In the United States thyroidectomy is a frequently performed surgery by both general and head and neck surgeons. Even the most experienced thyroid surgeon, however, has probably received a pathology report stating that an incidental parathyroid gland or parathyroid tissue was found in the submitted thyroidectomy specimen. The aim of this report is to explore some of the pathologic and clinical characteristics of unintentional parathyroidectomy during thyroidectomy. A retrospective review was performed of thyroidectomies performed at the University of California, Los Angeles, Center for the Health Sciences between 1989 and June 1998 which had pathology reports showing parathyroid tissue contained within the thyroidectomy specimen. This excluded any tissue submitted separately to be evaluated for parathyroid tissue and parathyroid tissue removed unintentionally during a thyroidectomy for a different procedure such as a laryngectomy or surgery for parathyroid disease. The pathology slides were reviewed to determine the incidence of unintentional parathyroid tissue removal, the size of the parathyroid tissue found within the thyroid specimen, the location of the parathyroid tissue (extracapsular, intracapsular, intrathyroidal), and whether this unintentional parathyroidectomy during thyroidectomy caused clinical consequences. Four hundred fourteen applicable thyroidectomies were performed during this time with 45 (11%) discovered cases of unintentional parathyroidectomy during thyroidectomy. Twenty-five (56%) cases were discovered during thyroidectomy for benign disease, and 20 (44%) during thyroidectomy for malignant thyroid disease. All the parathyroid tissue was normal and was found in extracapsular (58%), intracapsular (20%), or intrathyroidal (22%) locations. Of these 45 cases, recurrent laryngeal nerve paralysis was found only in two patients who had the nerve resected intentionally during the thyroidectomy, and none of the patients developed permanent hypocalcemia. Incidental parathyroid gland tissue was reported in 11% of the thyroidectomies performed in our series, without the clinical consequence of hypocalcemia. The majority (78%) of this parathyroid tissue was found in the extracapsular and intracapsular locations; therefore it is possible that these parathyroid glands may be identified and preserved with more meticulous inspection of the thyroid capsule during and after thyroidectomy to decrease the incidence of unintentional parathyroidectomy during thyroidectomy in the future.