Women suffering from infertility in association with mostly mild endometriosis were subjected to vaginal sonography of uterine peristalsis during the menstrual period, the early, mid- and late follicular phases, and the mid-luteal phase of the menstrual cycle. The data obtained were compared with those of healthy controls. Women with endometriosis displayed a marked uterine hyperperistalsis that differed significantly from the peristalsis of the controls during the early and mid-follicular and mid-luteal phases. During the late follicular phase of the cycle, uterine peristalsis in women with endometriosis became dysperistaltic, arrhythmic and convulsive in character, while in controls peristalsis continued to show long and regular cervico-fundal contractions. Hysterosalpingoscintigraphy during the early, mid- and late follicular phases revealed that hyperperistalsis in the early and mid-follicular phases of patients with endometriosis resulted in a dramatic increase in the transport of inert particles from the vaginal depot, through the uterus into the tubes and also into the peritoneal cavity. During the late follicular phase of the cycle, the dysperistalsis observed in women with endometriosis resulted in a dramatic reduction of uterine transport capacity in comparison with the healthy controls. We consider uterine hyperperistalsis to be the mechanical cause of endometriosis rather than retrograde menstruation. Dysperistalsis in the late follicular phase of patients with endometriosis may compromise rapid sperm transport. Uterine hyperperistalsis and dysperistalsis are considered to be responsible for both reduced fertility and the development of endometriosis.