Recurrence of hyperparathyroidism after a parathyroidectomy requires a review of previous surgical procedures and a review of the pathologist's report on the parathyroids. Were the usual four glands found, and where were they? Were any glands verified in the thyroid, thymus, anterior or posterior mediastinum, or above the thyroid? Which glands were removed or verified by frozen section examination? Every effort should be made to localize the parathyroids prior to any reoperation. Computed tomography, magnetic resonance imaging, radionuclide (sestamibi), and ultrasound scans may be useful for identifying large tumors. However, selective venous sampling with hormonal assays may be the last and best diagnostic option.