As a consequence of multiple follicular growth during ovarian stimulation for in-vitro fertilization (IVF), follicles of varying sizes often yield oocytes that vary in maturity and morphology of the oocyte-cumulus-corona complex. The objective of this prospective study was to explore the relationship between follicular fluid aspirate volume and the oocyte's developmental potential in an IVF treatment cycle. In total 9933 follicles were studied from 400 patients who underwent 535 consecutive IVF treatment cycles at St James's University Hospital, Leeds, UK, between February 1995 and February 1996. The volume of each individual follicle aspirated was recorded and related to the probability of obtaining an oocyte, its fertilizing capacity, the cleavage rate and the quality of embryos derived. We found no statistically significant difference in oocyte recovery rates between follicles with an aspirate volume < or = 1 ml and follicles with a volume > 1 ml. Although oocytes obtained from follicles with an aspirate volume > or = 1 ml showed a significantly lower fertilization rate, they went on to cleave at the same rate as oocytes obtained from larger follicles and resulted in embryos of comparable quality. Furthermore, there was no statistically significant difference in the implantation, clinical pregnancy or live birth rates per cycle between embryos derived from follicles with an aspirate volume < or = 1 ml and those derived from follicles with an aspirate volume > 1 ml. We conclude that follicular size and the oocyte's developmental potential in the stimulated ovary are not closely related and can be independent. This is in contrast to the Graafian follicle and the pre-ovulatory oocyte in the natural cycle.