Parathyroid scintigraphy and high-resolution ultrasonography are frequently used as preoperative localization procedures in primary hyperparathyroidism. However, when thyroid disease coexists, their diagnostic accuracy is probably abated. DESIGN: 56 patients with primary hyperparathyroidism were prospectively evaluated with parathyroid scintigraphy (with either thallium or technetium-99m agents or both) and 44 of them were also evaluated with ultrasonography. RESULTS: 33 patients (59%) had coexistent thyroid disease. Upon operation, 48 patients were found to have a solitary parathyroid adenoma and were all cured. One patient had a carcinoma and 7 had multiglandular parathyroid disease. Regarding solitary lesions, the sensitivity of parathyroid scintigraphy with Tc-agents was 97% and thallium 78%, while that of ultrasonography was 74%. The false positive rate was 2.6%, 18% and 22%, respectively. Concomitant thyroid disease had a non-significant effect on the results of parathyroid scintigraphy and ultrasonography. The efficiency of both modalities in diagnosing multiglandular disease was low (only 3/7 patients, 43%). CONCLUSIONS: Parathyroid scintigraphy, in conjunction with Sestamibi or Tetrofosmin, constitutes the most sensitive localizing technique as regards solitary lesions. Ultrasonography is also useful in confirming scintigraphic findings, offers more precise anatomic information, and is valuable in the evaluation of concomitant thyroid disease. The complementary use of parathyroid scintigraphy and ultrasonography is beneficial and efficacious in areas with high prevalence of thyroid disease. The value of these modalities is considerably lower in multiglandular disease.