Objective:To investigate the expression of FasL and its significance in thyroid adenocarcinomas. Methods: Immunohis-tochemical staining(S- P method) was used to detect expression of FasL in formalin - fixed and paraffin - embeded tissues from 27 cases of thyroid papillary adenocarcinomas(TPA)

20 medullary carcinomas(MC)and 12 normal thyroid tissues. The relationship between the thyroid adenocarcinomas and the positive rates of FasL expression was analyzed. Results: FasL had various degrees of expression in TPA.The positive rate of high FasL expression was 81. 8% in group B

31. 2% in group A

with a significant difference of P<0. 05. In some specimens

FasL immunoreactivity was stronger at the peripheral edge of the carcinoma

which was in close contact with adjacent normal tissues. The positive rate of strong FasL expression in MC were 95% . Gonclusion :Our results suggest that high levels of FasL expression in TPA are related to tumor aggressiveness; also thyroid carcinoma cells ascape immune surveillance by the expression of FasL; these findings suggest that the expression of FasL in TPA may contribute to further metastases in the surrounding tissues. FasL expression in MC is not associated with infiltration of the thyroid capsule or recurrence after surgery and metastases.