High-frequency (10-MHz) sonography demonstrated a cervical mass or lymphadenopathy, or both, during postoperative follow-up of 52 patients who had undergone surgery for thyroid cancer. Percutaneous biopsy with ultrasonographic (US) guidance was performed in all 52 masses, 44 of which were nonpalpable. Malignant cells were obtained in 29 biopsies, and the results of 20 biopsies were negative, yielding benign lymphocytes only. Results in three biopsies were nondiagnostic due to hypocellular specimens. Therefore, 94% of biopsy results (49) of 52) were confidently assigned as either positive (56%) or negative (38%) for malignancy. There were no complications. High-frequency sonography can demonstrate clinically occult thyroid bed tumor recurrence and lymph node metastases. US-guided biopsy is an accurate and safe technique to confirm or exclude malignancy in patients at high risk of recurrence of thyroid cancer.