PURPOSE: To assess the prevalence and the risk of malignancy in thyroid nodules which were cytologically diagnosed as follicular neoplasm by fine-needle aspiration (FNA) cytology. METHODS: All the patients who had undergone surgery with cytological diagnosis of follicular neoplasm from January 1996 through December 2001 in Asan Medical Center were studied retrospectively. Patients' and nodule characteristics were analyzed for factors associated with the presence of cancer. RESULTS: Two hundreds and fifteen patients (196 females, 19 males) were included and their mean age was 39.4 years (range: 12~76). About half of the patients (102 out of 215, 47.4%) had malignancy with 29 papillary carcinomas, 57 follicular carcinomas, 15 Hürthle cell carcinomas, and 1 medullary carcinoma. Previously suggested factors associated with risk for malignancy such as male gender, large tumor size (>4 cm), or age of patients (>45 years), were not associated with increased risk. Diagnosis of Hürthle cell neoplasia on FNA was also not associated with increased risk. Only the extreme age of the patients (below 20 or above 60 years) was associated with increased risk for malignancy. CONCLUSION: By our hands, prevalence of carcinoma in thyroid nodule patients with cytological diagnosis of follicular neoplasm was much higher than those reported. Clinical characteristics such as male gender, age, and nodule size, are not useful predictors for the presence of malignancy. Thyroid nodules with cytological diagnosis of follicular neoplasm warrant immediate surgery without any further work up.