The authors evaluated computed tomography (CT), ultrasound (US), technetium/thallium scintigraphy, and magnetic resonance (MR) imaging as localization procedures in 53 patients with proved parathyroid adenomas and previous unsuccessful parathyroid surgery. CT had the highest overall sensitivity (47%), followed by US (36%) and scintigraphy (27%). There is still too little data to assess MR imaging. Adenoma size affected the sensitivity of CT, scintigraphy, and MR imaging but not US. When all three studies were used, at least one study depicted a lesion in 78% of patients, but definitive localization (two positive studies) was achieved in only 31%.