Mediastinal parathyroid tumors are a frequent cause of failed parathyroid operations. I therefore reviewed my experience with 285 consecutive patients treated surgically for hyperparathyroidism from January 1981 to Dec 31, 1986. Two hundred eighty-eight operations were performed on these patients (246 primary, 38 secondary, one error in diagnosis, and 53 reoperations). Mediastinal parathyroid tumors were present in 64 (22%) of the entire group of 285 patients with hyperparathyroidism, and 20 (38%) of 53 patients requiring reoperation for persistent or recurrent hyperparathyroidism. Fifty-two parathyroid tumors were situated in the anterior mediastinum and 12 were found in the posterior mediastinum; 56 of the mediastinal tumors were removed via a cervical approach. In four patients the mediastinal tumor was a fifth histologically documented parathyroid gland. Mediastinal tumors were identified by preoperative localization studies (ultrasonography, three [16%]; thallium-technetium, five of 17 [29%]; computed tomography, eight of 14 [57%]; magnetic resonance imaging, three of seven [43%]; and selective venous catheterization for parathyroid hormone, ten of 11 [91%]). Awareness of the relatively high occurrence of mediastinal tumors (52 anterior, 12 posterior) should decrease the risk of failed parathyroid operations.