Developmental competence of in vitro matured human oocytes is dependent on the morphology of the cumulus-oocyte complex (COC) just after collection from the follicle. We postulated that COCs categorized as having poor morphology (two or fewer less than two layers of cumulus cells) would not secrete a sufficient amount of matulation factors, resulting in low developmental competence of the matured oocytes. In the present study, the level of progesterone secreted from good morphology COCs with three or more layers of cumulus cells, 39.2 ± 12.8 ng/ml (n=31), was significantly higher than that secreted from poor morphology COCs (9.65 ± 1.34 ng/ml, n=22). The addition of 20 ng/ml progesterone to in vitro maturation culture of the poor morphology group significantly improved the fertilization ability of the oocytes. The rates of development to the morula and blastosyst stages were also increased by progesterone, however the differences were not significant. In conclusion, the secreted level of progesterone during in vitro maturation of human COCs was dependent on the number of cumulus cells attached to oocyte. When an oocyte is surrounded by two or fewer 2 layers of cumulus cells, the addition of progesterone to FSH- and hCG-containing medium appears to be a useful method for obtaining an oocyte with a high developmental competence.