Daily changes in the thickness and texture of the endometrium were assessed by transvaginal sonography in 15 natural (cycles 'cycle-A) and 24 stimulated cycles (cycle-B: hMG-hCG cycles; hCG 10,000 IU was injected on the day after the dominant follicle reached 18mm in three-directional radius, n = 7) (cycle-C: hCG 3,000 IU injections on the 5th, 9th and 13th day after hCG 10,000 injection were added to cycle-B, n = 10) (cycle-D: hCG 1,000 IU instead of 3,000 IU of cycle-C, n = 7). The duration of the luteal phase was 15, 9, 17 and 18 days in cycle-A, cycle-B, cycle-C and cycle-D, respectively. The thickness of the endometrium increased lineally until ovulation day (D +/- 0) (thickness: 10.7mm) in cycle-A, but in stimulated cycles it revealed a significantly rapid increase until D-4 (10.9mm) and then reached a plateau. The ratio of the thickness of the hyperechoic area to the total thickness of the endometrium reached 100% (D + 9) in cycle-A. The ratio reached only 72% (Max, D + 4) in cycle-B, but the effect of luteal support was shown to reach 89% and 85% (Max, D + 7) in cycle-C and cycle-D. The serum progesterone level in stimulated cycles revealed a rapid increase until D + 5, but it had an unstable pattern in cycle-C and cycle-D. It was shown that luteal support was necessary to offset the luteal phase defect caused by stimulation with hMG.