['Concomitant thyroid nodules are the most common reason for false-positive ultrasonography (US) results in primary hyperparathyroidism. The aims of this prospective clinical study were to evaluate false-positive US results according to the characteristics of concomitant thyroid nodules and to determine which characteristics of thyroid nodules are important.', 'This prospective study included 120 consecutive patients with primary hyperparathyroidism. The patients were divided into 2 groups according to preoperative US results. Group 1 consisted of 32 patients with false-positive US results and group 2 consisted of 88 patients with true-positive US results.', 'The risk for false-positive US result was increased 25-fold for patients with parathyroid adenoma weight of more than 500 mg (odds ratio [OR], 25; 95% confidence interval [CI], 8.6-74.5), 75-fold for more than 1 posteriorly located thyroid (OR, 75; 95% CI, 19.3-293.4), 358-fold for the presence of exophytic thyroid nodules (OR, 358; 95% CI, 42.3-3036), and 423-fold for the presence of posteriorly located thyroid nodules (OR, 423; 95% CI, 49-3662).'] Do the characteristics of concomitant thyroid nodules cause false-positive ultrasonography results in primary hyperparathyroidism?