We reviewed histological and immunohistochemical findings of the endometrium in 28 cases of ectopic pregnancy and 11 cases of intrauterine pregnancy without chorionic villi or syncytiotrophoblast. 1) Twenty cases(71.41/o) of ectopic pregnancy revealed gestational patterns and 8 cases(28.6%) showed non-gestational patterns, which were menstrual phase in 3 cases, proliferative phase in I case, early secretary phase in 3 cases and mid secretary phase in 3 cases, respectively. Implantation sites were present in 36.40/o of intrauterine pregnancy. 2) Endometrial spiral arterioles tend to be more prominent with frequent intimal proliferation and thickening of the wall in intrauterine pregnancy than in ectopic pregnancy although it was not statistically significant(p=0.271). 3) Deposition of fibrinoid material in the endometrium was present in 72.7% of intrauterine pregnancy and 25% of ectopic pregnancy. Thrombosis was present in 72.7% of intrauterine pregnancy and 5% of ectopic pregnancy. Hyalinized vessels were also present in 90.9% of intrauterine pregnancy and 200/o of ectopic pregnancy. These were statistically significant(p=0.0002, 0.0209 and 0.0004), but not diagnostic. 4) On immunohistochemical study for intrauterine pregnancy, the rates of positive reaction to human placental lactogen, cytokeratin and human chorionic gonadotropin were 45.5%, 45.5% and 9%, respectively. We concluded that HFIL and cytokeratin are reliable and sensitive markers for implantation site.