OBJECTIVE: To characterize endometrial development in unexplained and tubal factor infertility. DESIGN: Prospective study of 20 women with unexplained infertility, 22 with tubal factor infertility, and 21 fertile controls in the midproliferative, periovulatory, and midluteal phases of the menstrual cycle. SETTING: Reproductive Medicine Department of St. Mary's Hospital, Manchester, United Kingdom. PATIENT(S): Women awaiting assisted conception. INVESTIGATION(S): Serum hormone assays, transvaginal ultrasound, Doppler, and midluteal endometrial biopsies. MAIN OUTCOME MEASURE(S): Serum levels of E2, P, and LH, endometrial ultrasound morphometry, uterine and subendometrial artery Doppler, and endometrial histology and biochemistry. RESULT(S): Women with unexplained infertility demonstrated significantly reduced uterine artery flow velocity in all phases, significantly elevated uterine and subendometrial artery impedance in the periovulatory and midluteal phases, and significantly reduced endometrial texture in the midproliferative phase. Women with tubal factor infertility demonstrated significantly reduced uterine artery flow velocity, without a concomitant increase in impedance, and significantly greater expression of endometrial glandular and luminal keratan sulphate. CONCLUSION(S): Unexplained infertility is associated with a profound impairment of endometrial perfusion that might be amenable to treatment by perfusion enhancers. Tubal factor infertility is associated with endometrial developmental defects that might be corrected by salpingectomy. Endometrial ultrasound and Doppler studies are likely to become a vital tool in the investigation of infertility.