We report a patient with a papillary carcinoma of the thyroid occurring within a hyperfunctioning adenoma. This patient underwent surgery on the basis of a positive fine-needle aspiration biopsy even though the 123I scan showed a hyperfunctioning nodule suppressing the rest of the gland. To avoid overlooking a thyroid carcinoma in a patient with a thyroid nodule, fine-needle aspiration biopsy should be performed routinely and a surgical procedure should be done when indicated by cytological data, regardless of scan findings.