Fine needle aspiration biopsy (FNAB) of the thyroid is limited in distinguishing hyperplastic nodular goiter from true follicular neoplasm and in separating follicular adenoma from follicular carcinoma. The present study was done to evaluate if the morphologic and morphometric investigations and silver staining of nucleolar organizer regions (NORs), either alone or in association would help to differentiate the thyroid follicular diseases. 40 FNAB smears of thyroid follicular diseases, histopathologically diagnosed as nodular goiter, lymphocytic thyroiditis, follicular adenoma and follicular carcinoma, were analyzed using the standard cytological exam, quantitative analysis and NORs assessment. The qualitative evaluation, correlated with the numerical results obtained from the quantitative analysis, revealed that the cellular pattern, mean nuclear diameter and NORs area are valuable criteria in the diagnosis of the benign and malign follicular lesions, respectively. The results attained through morphometry increase the sensibility and the specificity of FNAB in the diagnosis of thyroid follicular carcinomas.