Human thyroid peroxidase (hTPO) is a membrane protein with a key role in the thyroid hormones synthesis. Loss of hTPO was described in malignant tumours of the thyroid gland. hTPO was tested as a marker of malignancy. Immunohistochemical study of hTPO in 321 thyroid lesions (45 malignant tumours, 72 benign tumours, 199 benign non-tumours lesions, and 5 normal thyroid glands) is presented. The sensitivity of hTPO in predicting malignancy in thyroid is 64%, and the specificity is 87%. Thus, hTPO is of limited value in the diagnosis of thyroid malignancy. The authors discuss the role of hTPO as a marker of differentiation.