US-guided fine needle aspiration cytology is currently the best diagnostic tool for thyroid nodules. The aim of this research was to make a detailed and objective determination of the morphological characteristics of cells in cytological smears in an attempt to distinguish benign from malignant follicular tumors. The research included 62 patients with cytologically diagnosed follicular or oncocytic tumors, and 15 patients with nodular hyperplasia. Echographic findings were divided into three groups: isoechogenic, hypoechogenic and hyperechogenic nodules. We analyzed the cellularity of the smear, cohesion between follicular cells, acinar formations, bare nuclei, characteristics of the nucleus and the cytoplasm, and the presence of colloid. The statistical analysis of cytological parameters has indicated that none of the cytological parameters alone is discriminating enough between non-tumor and tumor changes, or benign and malignant follicular thyroid nodules. The analysis of age, sex, nodule size and ultrasound findings has not shown the correlation between any of these parameters with the malignant or benign follicular tumors. The cytological analysis of the smears for patients with follicular tumors, in combination with clinical data and other diagnostic methods, contributes to more precise diagnostics, but is not sufficient for the differentiation between benign and malignant follicular tumors.