A total of 293 oocytes that failed to develop pronuclei after insemination in vitro with apparently normal, fertile spermatozoa were obtained from 87 women undergoing therapeutic in-vitro fertilization. The oocytes were investigated cytogenetically to determine the incidence of immaturity and chromosomal abnormalities. Cytogenetic examination was possible in 81% of the preparations, in which immaturity and chromosomal abnormalities were present in 29.5 and 58.7% respectively.