Primary oocytes recovered from small and growing follicles of > or = 3 mm in the ovaries of untreated women, can be matured in vitro, will fertilize and develop in vitro, and when transferred to the patient, develop to term. However, the implantation rate of cleaved embryos has been disappointingly low and when embryos are allowed to develop beyond the 4-cell in vitro, retardation of development and blockage is frequently observed, with relatively few embryos developing to blastocysts. We have devised new culture systems for human embryos to enable high rates of development of in-vivo matured oocytes to blastocysts within 5-6 days of culture, and high implantation rates of these blastocysts when they are transferred to the patients' uterus. These culture systems are now being used for in-vitro matured oocytes. In order to determine whether embryo developmental competence could be improved, a number of factors were examined. Treatment of patients with pure follicle stimulating hormone (FSH) early in the follicular phase, or treatment with oestrogen prior to oocyte recovery, had no apparent effect on any parameters of oocyte developmental competence. There was no indication that a medium made specifically for human oocyte maturation improved oocyte developmental competence. Nuclear and cytoplasmic changes in oocytes matured in vitro appear to be similar to that in vivo, although some lack of synchronization in completing maturation is evident. It is possible that follicles of < 10 mm diameter in the human contain developmentally-incompetent oocytes. However, the development to term and birth of normal babies from germinal vesicle stage oocytes recovered from small follicles and matured in vitro, suggests that further research will identify the factors necessary to improve embryo developmental competence. The application of immature oocyte collection (IOC) and in vitro maturation (IVM) as an alternative to ovulation stimulation with high doses of gonadotrophins for in-vitro fertilization (IVF), remains a priority for research in human medicine.