Intraoperative decision making during thyroid surgery based on the results of preoperative needle biopsy and frozen section. Prognostic factors in well-differentiated thyroid cancer are age of the patient and grade, size, distant metastasis, and extracapsular spread of the disease. However, the surgeon is often not sure about the pathologic diagnosis of thyroid nodules. The accuracy of preoperative studies, such as ultrasonography and thyroid scanning, is limited. The most cost-effective test is fine-needle aspiration, the accuracy of which exceeds 80% in most series. However, a large group of nodules exist for which aspiration cytologic studies are considered to be either suspicious or indeterminate. The decision about the extent of thyroidectomy may be difficult in these patients. Intraoperative frozen section may help the surgeon to distinguish benign from malignant lesions, but as in fine-needle aspiration, the major problem is the distinction between follicular adenoma and follicular carcinoma. The frozen section diagnosis of follicular adenoma was changed to follicular carcinoma in one third of the cases (13 of 38 cases). The decision about the extent of thyroidectomy in patients with follicular adenomas was based on other prognostic factors, such as age and sex of the patient and the size of the nodule. The accuracy of frozen section diagnosis was 95%. Our experience suggests that decisions regarding the extent of thyroidectomy can best be made by preoperative fine-needle aspiration with confirmation by frozen section diagnosis in equivocal cases. 