Retrograde seeding of endometrium to the peritoneum during menstruation is an accepted theory to explain the development of endometriosis. It could be demonstrated that endometrial glands can be found, in a high percentage of cases, in the peritoneal cavity following flushing of the uterus and tubes performed before as well as after curettage, at any stage of the menstrual cycle, and it is, therefore, suggested that insufflation after curettage or during menstruation may cause endometriosis. The purpose of this study was to investigate whether or not tubal insufflation with CO2 may also cause dissemination of endometrial cells to the peritoneal cavity. But no endometrial tissue, fragments or cells, were detected in the fluid samples which were aspirated from the peritoneal cavity before as well as after CO2 insufflation. CO2 insufflation apparently does not cause spreading of endometrial tissue to the peritoneal cavity.