Objective   To investigate the possible roles of Cyclin A and P21 in the pathogenesis and development of endometriosis.    Methods   Twenty-five specimens of eutopic endometrium and twenty-eight specimens of ectopic endometrial tissues were obtained from 28 patients with endometriosis (observation group). Thirty-five specimens of endometrium were also obtained from women with other benign diseases (control group). Cyclin A and P21 were detected by Western blotting

RT-PCR and immunohistochemistry.    Results   The expression level of cyclin A mRNA in ectopic endometrium were (1.24±0.10) at the proliferative phase and (1.33±0.21) at the secretory phase

significantly higher than that in the eutopic endometrium tissues (0.72±0.26

0.42±0.22)

control group (0.68±0.09

0.35±0.06) (P0.05). Its protein expression was (0.95±0.10) at the proliferative phase

and (0.91±0.18) at the secretory phase

which were significantly higher than those of the eutopic endometrium (0.53±0.08

0.27±0.11)

and control group (0.47±0.11

0.22±0.09) (P0.05). The expression level of Cyclin A in proliferative phase and secretory phase of the eutopic endometrium had no significant difference compared with those of the control group (P0.05). The expression level of P21 mRNA in control group were (0.21±0.01) at the proliferative phase and (0.54±0.35) at the secretory phase

significantly higher than that in the eutopic endometrium tissues (0.09±0.06

0.28±0.02) and in the ectopic endometrium (0.06±0.01

0.13±0.00) (P0.05). The expression level of P21 protein in control group were (0.74±0.13) at the proliferative phase and (1.13±0.15) at the secretory phase

significantly higher than that in the eutopic endometrium tissues（0.59±0.21

0.62±0.29）and in the ectopic endometrium（0.40±0.36

0.43±0.33） (P0.05). The expression level of Cyclin A in proliferative phase of the control group was significantly higher than secretory phase. The expression level of P21 in secretory phase of the control group was significantly higher than proliferative phase. But the expression level of Cyclin A and P21 in the proliferative phase and secretory phase of the ectopic endometrium had no significant difference (P0.05). In the ectopic endometrium they had negatively spearman correlation （r=-0.738

P0.01).    Conclusion   The abnormality expression of Cyclin A and P21 in the ectopic endometrium may be involved in the proliferation of endometrium in endometriosis.