Objective To investigate the expression of histone deacetylase1 (HDAC1) in the eutopic and ectopic endometrium of women with endometriosis. Methods The expression of HDAC1 in the eutopic and ectopic endometrium of 20 cases of women with endometriosis, and the expression of HDAC1 in the endometrium of 20 cases with hysteromyoma were mesured by immunohistochemical technique and Western blotting. Results HDAC1 was located in the nuclei of epithelial and stromal cells. The expression level of HDAC1 in eutopic endometrium was significantly higher than that in the control group (EM>P/EM><0.01). In Western blotting, semiquantitative analysis showed that the relative expression of HDAC1 protein in eutopic and ectopic endometrium were significantly higher than that in the control group (2.67±0.69；2.55±1.36 vs 1.63±0.93, EM>P/EM><0.01，EM>P/EM><0.05). But there was no significant difference between eutopic and ectopic endometrium (P>0.05). Conclusion The higher expression of HDAC1 in eutopic and ectopic endometrium with endometriosis indicates that it may play an import