Objective: To describe the propagation direction of the subendometrial myometrial contractile wave in menstrual phase sonographically assessed in patients with and without endometriosis. Design: Prospective study. Setting: Patients and normal volunteers in a university hospital. Patients: Sixteen women with (study group) and 21 without (control group) endometriosis. Interventions: Patients were examined by transvaginal sonography (midsagittal plane of the uterus) and recorded on videotape for a 5-minute period. Main Outcome Measure: Direction of propagation of contraction waves. Results: The endometriosis group showed a predominant retrograde pattern whereas the control group showed a normal antegrade. Conclusion: This abnormal cervix-to-fundus myometrial activity could well increase the amount of peritoneal shedding of endometrial menstrual debris, increasing the probabilities of its heterotopic implantation.