The degree and normality of nuclear maturation were assessed with the fluorochrome Hoechst 33342 in two groups of inseminated human oocytes that had failed to undergo fertilization. Group 1 consisted of 67 oocytes from 27 patients, each of whom had at least two other oocytes that had been fertilized and had cleaved. Group 2 consisted of 65 oocytes from 14 patients, none of whose oocytes had been fertilized. In group 1, 52.3% of the oocytes were found to be immature (germinal vesicle stage or metaphase-telophase I), whereas in group 2 only 26% were found to be immature. Thus oocyte nuclear immaturity was the major cause of fertilization failure when companion oocytes were fertilized. When no oocytes of a patient were fertilized, most oocytes were found to be mature, so other factors, such as sperm dysfunction or zona binding abnormalities, must account for most of the fertilization failure in this group of patients.