In this prospective study, we compared the effect of delayed inseminations on fertilization, cleavage and pregnancy rates in two groups of patients. In one group, the ovarian stimulation was performed with a clomiphene citrate/human menopausal gonadotrophin/human chorionic gonadotrophin (CC/HMG/HCG) protocol. The other group was pre-treated with gonadotrophin-releasing hormone agonist (GnRHa) and ovarian stimulation was carried out with an HMG/HCG protocol. Under both protocols, a delay of 2, 4 or 6 h in insemination showed no significant differences in the fertilization, cleavage or pregnancy rates. To find out which type of oocyte has the potential for better fertilization, cleavage and implantation, a simple oocyte classification scheme is proposed. In the GnRHa/HMG group, 9% post-mature, 90% mature and only 1% immature oocytes were retrieved. The post-mature oocytes showed a tendency towards reduced fertilization when insemination was delayed. The mature and slightly immature oocytes fertilized equally well when spermatozoa were added 2, 4 or 6 h after retrieval. Similarly, no significant difference was observed in the cleavage (80%) or fragmentation (20%) rates of these oocytes. The pregnancy rates after inseminations delayed for 2, 4 and 6 h were 14, 27 and 26%, respectively. Though these figures were not statistically significant, the 4- and 6-h groups in both the IVF and zygote intra-Fallopian transfer treatments showed a slightly improved pregnancy rate compared to the 2-h group. An insemination delay of 4 h is advocated on a routine basis.