Endometrial vascular changes similar to atherosclerosis of toxemia of pregnancy were described and graded in 217 consecutive endometrial biopsies of known early intrauterine pregnancy. Severe vascular changes were found in 23.5% of cases. Control material consisting of endometrial biopsies of patients with known cases of tubal ectopic pregnancy and various non-pregnancy menstrual disorders showed minimal or no changes except in one case. A parallel study of Aria-Stella phenomenon in 110 cases of uterine pregnancy showed significant changes in only 3.6% of patients. It is obvious that in these cases of positive Arias-Stella findings, the possibility of an extrauterine pregnancy could not be discarded on histologic grounds alone. The authors suggest the use of the vascular changes of the more severe histologic degree as described in this article as a positive or strongly suspicious marker for intrauterine pregnancy whenever this is needed.