Abstract: Objective : To investigate the clinico-pathological significance of thymosin β10(Tβ10)in human breast cancer. Methods : 113 formalin fixed, paraffin -embedded breast tissue samples, in-cluding 10 fibrocystic lesions, 76 primary carcinomas and 27 metastatic carcinomas in axillary lymph nodes, as well as two human breast cancer cell lines (differing in their estrogen receptor expression,histological grade and metastatic potential), were analyzed by immunohistochemical analysis with a specific antibody for Tβ10. E5 and c-erbB2 proteins were also examined in all 76 primary breast cancers.Results: Tβ10 was found in all the carcinoma cases and the staining pattern was cytoplasmic, whereas the benign fibrocystic lesions showed very weak staining or did not stain at all . Also, the intensity of staining in the in situ carcinomas with microinvasion was markedly weaker than the infiltrating and metastatic lesions. In both clinical cases and tumor cell lines, the expression of Tβ10 was found to be increased with the rising differentiation grade of carcinomas. Although the staining intensity is stronger in the metastatic lesions compared with the primary carcinomas, there is no statistically significant difference (P>0.05). No correlation was found in Tβ10 expression level with estrogen receptor status or cerbB2 status. Conclusions : Although minute amounts of Tβ10 are present in fibrocystic lesions, the expression of Tβ10 was found to be markedly increased during malignant transformation and invasion of the human breast cancers, Thus Tβ10 might be one oi the new markers for diagnosing and preaicting prognosis of the human breast cancers.