A prospective study was undertaken to evaluate and compare magnetic resonance imaging (MRI) and ultrasound (U/S) in monitoring follicular and endometrial development during the normal menstrual cycle. Results of MRI, U/S, estradiol (E2), and progesterone were obtained from five ovulatory volunteers on days 4, 8, 12, 16, 20, and 24 of the cycle. On U/S, all the women had only one dominant follicle, whereas MRI was able to detect a secondary cohort of follicles in three of five volunteers. Endometrial development on U/S and MRI showed similar patterns of growth with an increase on MRI from 5.8 ± 1.1 mm on day 4 to a mean peak of 10.3 ± 1.7 mm on day 24. A distinct feature of MRI was the demonstration of a junctional zone (JZ), which has a pattern of growth that resembles that of the endometrium, with accelerated growth from day 8 to day 16 (5.1 ± .7 mm to 6.7 ± .7 mm). The JZ corresponds anatomically to the distribution of the arcuate vessels and may therefore represent changes in blood supply to the endometrium. MRI, similar to U/S, is noninvasive and does not involve any radiation exposure but provides new information on uterine changes in the normal cycle. At present, its clinical applications are limited.