Blood was found in the peritoneal fluid in 90% of women with patent tubes at laparoscopy during perimenstrual time. If the fallopian tubes were occluded, then only 15% of patients had evidence of blood in the pelvis. Also, 90% of patients with endometriosis and eight of nine women on oral contraceptives had bloody fluid during the menstrual period. The present observations indicate that retrograde menstruation through the fallopian tubes into the peritoneal cavity is a very common physiologic event in all menstruating women with patent tubes.