BACKGROUND.: A significant number of thyroid fine-needle aspiration cytology (FNAC) cases yield inconclusive results. The recent National Cancer Institute guidelines and those published by other societies are important contributions to standardizing the diagnostic approach. Nevertheless, there are significant issues in the application of guidelines and the evaluation of their clinical efficacy. Data from individual departments can be useful in demonstrating the role of standardized reporting. METHODS.: The authors followed 529 consecutive cases with inconclusive thyroid FNA results that were analyzed in a single laboratory in Western Australia. In that laboratory, standardized reporting in categories has been in place for a decade, and inconclusive cases have been subdivided into indeterminate and atypical groups. Follow-up data was obtained for 341 indeterminate cases (17.2% of total thyroid FNA accessions) and for 188 atypical cases (9.5% of accessions). RESULTS.: In total, 127 nodules with atypical results (67.6%) underwent surgical excision compared with 131 nodules with indeterminate results (38.4%; P < .0001). In 96 excised nodules with atypical results (75.6%), the excised specimens were identified as neoplastic compared with 61 excised nodules with indeterminate results (46.6%; P < .0001). In addition, 31 excised nodules with atypical results (24.4%) had a malignancy proven compared with 17 excised nodules with indeterminate results (13%; P < .05). In addition, 51 of 82 repeat FNAs (62.2%) among patients who had indeterminate results yielded a more specific diagnosis compared with 2 of 9 repeat FNAs (22.2%) among patients who had atypical results (P < .05). CONCLUSIONS.: The routine subcategorization of patients who had inconclusive thyroid FNA reports into indeterminate and atypical groups resulted in statistically significant differences in the likelihood of neoplasia and malignancy. Patients who had indeterminate results were more likely to benefit from repeat FNAC than patients who had atypical results. The current results indicated that patients who fall into these 2 categories are likely to benefit from different clinical management protocols. Cancer Cancer Cytopathol 2010. © 2010 American Cancer Society.