OBJECTIVE: To establish the role of uterine contractions in retrograde menstruation with subsequent abdominal implantation of endometrial tissue. DESIGN: Controlled prospective study. SETTING: University hospital-based study. PATIENT(S): Infertile women with (n = 22) and without (n = 22) endometriosis. MAIN OUTCOME MEASURE(S): Frequency, amplitude, and basal pressure tone of uterine contractions; correlation of contractions with retrograde bleeding and presence of viable endometrial cells; and dysmenorrhea before and 3 and 24 months after surgery. RESULT(S): Compared with controls, patients with endometriosis had uterine contractions with higher frequency (22.73 +/- 5.66 osc/10 min vs. 11.09 +/- 3.26 osc/10 min), amplitude (20.83 +/- 3.94 mm Hg vs. 6.77 +/- 2.83 mm Hg), and basal pressure tone (50.14 +/- 16.30 mm Hg vs. 24.68 +/- 6.14 mm Hg). Dysmenorrhea was scored as 4.09 +/- 1.44 in patients with endometriosis and 0.86 +/- 1.42 in controls. Retrograde bleeding was found in 73% of patients with endometriosis vs. 9% of controls, and only 45% of patients with endometriosis had viable endometrial cells in the cul-de-sac. CONCLUSION(S): Endometriosis may result from abnormal myometrial contractility through tubal transportation, dissemination, and implantation of endometrial viable cells into the abdomen.