Galectin-3 is a protein of the lectin family that has been associated with neoplastic processes in various tissues. In the thyroid, expression of this protein has been described in differentiated follicular cancer, suggesting that the immunohistochemical study of galectin-3 may be a potential marker of malignancy in thyroid neoplasms. The confirmation of these results may represent an extremely useful tool for presurgical diagnosis and medical conduct. In this study, galectin-3 protein and mRNA expression were analyzed in the thyroid tissues from 87 patients with histomorphological diagnosis of multinodular goiter (MNG) (n = 24), follicular adenoma (n = 31), follicular carcinoma (n = 20), papillary carcinoma (n = 12), and five normal tissues. Galectin-3 protein expression was detected by immunohistochemical method in light, fluorescence, and confocal microscopy, using monoclonal antibody. Galectin-3 mRNA expression was detected by the RT-PCR method. Our results showed that the majority of carcinomas expressed galectin-3 protein (follicular, 90%; papillary, 100%). However, in contrast to the previously published data, benign lesions also expressed galectin-3 (adenoma, 45%; MNG, 17%). We further demonstrated by RT-PCR that thyroid tissues with diagnosis of adenoma and MNG-expressed galectin-3 mRNA. Although the galectin-3 immunostaining demonstrated a sensitivity of 93.8% in the identification of cancer, the accuracy in the distinction between benign and malignant tissues was 77.0%. This accuracy was even lower (68.6%) when the galectin-3 expression in follicular adenoma was compared with follicular carcinoma. Thus, the use of galectin-3 immunodetection as a molecular marker for thyroid carcinoma must be interpreted with caution, particularly in the differentiation between thyroid follicular carcinoma and follicular adenoma.