应用免疫组化方法检测了３７例乳腺癌、２５例良性乳腺疾病和１０例正常乳腺组织ｒａｓ癌基因产物Ｐ２１的表达水平。结果显示：乳腺癌Ｐ２１的阳性反应率（７０．３％）远高于良性乳腺疾病（８％）及正常乳腺（０％），且Ｐ２１表达水平随肿瘤体积、局部浸润和淋巴结转移的扩展而升高（Ｐ＜０．０１）。癌组织分化低者Ｐ２１表达水平亦较高（Ｐ＜０．０５）。提示Ｐ２１过度表达是乳腺癌预后不良的征象。 Immunohistochemistry was used to detect the expression of ras oncogene product P21 in 37 cases of breast cancer, 25 cases of benign breast disease and 10 cases of normal breast tissue. The results showed that the positive rate of P21 in breast cancer (70.3%) was much higher than that in benign breast disease (8%) and normal breast (0%), and the expression level of P21 was related to the expansion of tumor volume, local infiltration, and lymph node metastasis. Increased (P<0.01). The expression level of P21 was also higher in cancer tissue with low differentiation (P<0.05). It suggests that P21 overexpression is a sign of poor prognosis of breast cancer.