Objective: To assess the role of high-resolution ultrasonography in the preoperative evaluation of patients with hyperparathyroidism. Design: Retrospective cohort. Setting: Tertiary care university hospital. Patients: Three hundred seventeen patients with hyperparathyroidism who underwent preoperative assessment with ultrasonography and/or sestamibi scan and surgical treatment between October 2003 and October 2006. Main Outcome Measures: Descriptive statistics of localization imaging studies and detection of concomitant thyroid disease in patients presenting with hyperparathyroidism. Results: Ultrasonography correctly localized the parathyroid adenoma(s) in 148 (69.4%) of the 229 patients with these lesions. Sestamibi scans correctly localized the parathyroid adenoma(s) in 133 (58.1%) of the 229 patients. The agreement between the 2 imaging procedures was moderate (κ=0.23; 95% confidence interval, 0.12-0.36). Of 317 patients with hyperparathyroidism, 96 (30.3%) had clinically significant concomitant thyroid disease requiring partial or total thyroidectomy. Histopathologic examination revealed benign thyroid disease in 80 (83.0%) of the 96 patients and thyroid carcinoma in 16 (16.6%). Conclusion: Ultrasonography is a useful tool in the preoperative evaluation of patients with hyperparathyroidism both for localization of parathyroid adenomas and for the diagnosis of concomitant thyroid disease. ©2007 American Medical Association. All rights reserved.