Thin needle aspiration biopsies for cytology were performed upon 64 patients with hypofunctioning thyroid nodules. Surgical confirmation of the diagnoses was obtained in 41 of these patients. All 12 nodules with positive needle biopsies were found to be malignant at operation. Sixteen of 17 nodules with negative findings at biopsy were found to be benign; one was found to be malignant. Twelve nodules were in the questionable cytologic group, three were malignant and nine were benign. Twenty-three patients did not undergo operation after needle biopsy. Negative biopsy findings in these patients were instrumental in guiding decisions not to recommend operation. There was no morbidity after any of the 64 needle biopsies. An analysis of our results indicates that thin needle aspiration biopsy is a valuable diagnostic tool for the selection of thyroid nodules for surgical treatment. Its routine use on our service has increased the incidence of carcinoma in surgically explored cold thyroid nodules in the past year. The procedure was helpful not only in avoiding operation for some benign lesions but also in enabling the detection of a number of otherwise unsuspected malignant tumors. Objections which are usually raised to the large scale use of thin needle aspiration biopsies of thyroid nodules are not valid.