Several reports have shown decreased fertilization and implantation rates in high-responder patients to gonadotropins. In this cytogenetic study of unfertilized human oocytes, we have investigated possible reasons for this clinical evidence. Three groups of patients were established according to the number of oocytes retrieved: group 1 (n = 21), 1 to 5 oocytes; group 2 (n = 32), 6 to 10 oocytes; and group 3 (n = 35), greater than or equal to 11 oocytes. Patients in group 3 had lower estradiol levels per follicle developed, lower follicular volume, higher incidence of diploid oocytes, and higher rate of oocytes with an additional set of prematurely condensed sperm chromosomes of the presynthetic gap than groups 1 and 2. These results suggest that women with greater than or equal to 11 oocytes retrieved have a relative higher cytoplasmic immaturity that could explain the lower fertilization and implantation rates found in other studies.