Magnetic resonance (MR) imaging was performed on 21 women in different phases of the menstrual cycle, nine who used oral contraceptives and 12 who did not. Sagittal images in a dual spin-echo sequence were obtained to assess conspicuity and dimensions of the corpus uteri, cervix, vagina, and their component tissues. The endometrium, junctional zone, myometrium, cervical canal, fibrous stroma, and vaginal canal were seen in all cases and the vesicovaginal septum, rectovaginal septum, and levator ani in 90%, 86%, and 100%, respectively. The uterus was retrodisplaced in 19% of the women. In the nonpill group, endometrial width was significantly larger (P less than .025) in the secretory phase (mean, 5 mm) than in the follicular phase (3.1 mm). In the pill-using group, endometrial width was 1.1 mm in both phases, significantly smaller than in the nonpill group (P less than .025, follicular; P less than .0005, secretory). Junctional zone width was significantly smaller in the pill-using group (P less than .005). Qualitatively, the myometrium was relatively brighter on second echo images in the pill-using group, compatible with known edema that occurs with use of oral contraceptives. MR can reliably demonstrate gynecologic structure and anatomic changes that occur during the menstrual cycle and with use of oral contraceptives.