Ectopic implantation of the embryo in the tube opposite to the ovary containing the corpus luteum constitutes evidence of peritoneal or uterine transmigration of the egg. The frequency of this phenomenon was reinvestigated utilizing histopathologic confirmation of the side of the corpus luteum. A tubal pregnancy contralateral to the ovulating ovary was found in 28% of 67 cases, indicating that either the oocyte, the zygote or the embryo had entered the tube in which implantation took place from a medial site such as the peritoneal or the uterine cavity rather than directly from the ovulating ovary. Assuming that once in the medial site there is equal chance of entering either tube, it follows that in 56% of tubal pregnancies the egg has entered the tube from a midline location. Attempts to recover the oocyte from the tubes in normal women were successful in fewer than 5% of cases contralateral to the corpus luteum. It is concluded that tubal pregnancy is associated with a significant increase in the occurrence of transmigration of the egg.