Fine-needle aspiration biopsy is widely used in the diagnosis and management of the solitary thyroid nodule. It is the most accurate tool available and decreases the need for ultrasonography and thyroid scanning. In those patients who are selected for surgical treatment by fine-needle aspiration biopsy, it has been advocated as a guide to determining the extent of operation. Frozen section, which usually serves as the surgeon's guide, and fine-needle aspiration biopsy both have varying accuracy rates. Few direct comparisons have been made. We studied 198 aspirates in 198 patients who presented with a solitary nodule and had surgical excision. Fine-needle aspiration biopsy (198 cases) and frozen section (182 cases) were compared with the final histologic diagnosis. Accuracy rates for fine-needle aspiration biopsy and frozen section were 90 percent and 95 percent, respectively. Fine-needle aspiration biopsy detected 43 percent of the cancers and frozen section, 64 percent. There were no false-positive diagnoses with frozen section, but three cases with fine-needle aspiration biopsy. The false-negative diagnosis rate was 5 percent for frozen section and 8.5 percent for fine-needle aspiration biopsy. When the "other" diagnosis category was grouped with the "positive" diagnosis category and a single expert cytopathologist was used to read the cytology report, the sensitivity was increased to 80 percent whereas the accuracy was maintained at 83 percent. We believe that since there were no false-positive diagnosis using frozen section, it can reliably be used as a guide when it reveals malignancy. When the fine-needle aspiration biopsy diagnosis is "positive" or "other," it can guide operation, but only after carefully assessing the wording of the cytology report and the clinical situation. Each modality can provide information missed by the other. We continue to use them as complementary tools in the diagnosis and management of solitary thyroid nodules.