Traditionally the extent of thyroidectomy in patients with nodular thyroid disease has been based on peroperative frozen section examination (FS). Fine-needle aspiration biopsy (FNAB) and FS were evaluated with regard to the reliability to determine whether an operation for cancer is necessary. Both methods were performed in 240 patients operated for nodular thyroid disease and compared with the final histology on paraffin sections. Altogether 72 (30%) patients were found to have a malignant lesion on final histology. Only a malignant FNAB diagnosis and a malignant FS diagnosis were considered positive results for determining the extent of thyroidectomy. The test characteristics were equal: the sensitivity of FNAB and FS was 67%, the specificity 99%, and the accuracy 89%. The positive predictive value was 96% for FNAB and 98% for FS; the negative predictive values were 88% and 87%, respectively. Further analysis of the results indicates that FS is not necessary for patients with a malignant FNAB result. These patients should undergo a therapeutic operation for malignancy. When the FNAB result is uncertain, patients should undergo diagnostic surgery, and definitive surgery should be based on the final histology. Routine use of FS can be omitted.