Experimental evidence has shown that mice lacking the oestrogen receptor (ESR) gene are infertile with cystic ovaries and follicular arrest. In humans, several polymorphisms and mutations in the ESR gene have been identified. In this study we have analysed a common PvuII and a rare BstUI polymorphism in the ESR gene. Analysis was carried out on DNA samples from women undergoing ovarian stimulation for in-vitro fertilization (IVF) and embryo transfer and controls having at least one pregnancy. Comparisons were done between the three PvuII genotypes, concerning the mean numbers of follicles and oocytes and the mean ratios of follicles to oocytes harvested in two consecutive cycles. Significantly lower ratios were identified in the group lacking the PvuII polymorphism, compared with the groups with heterozygous or homozygous PvuII polymorphisms (P > 0.05 and P > 0.01 respectively). The rare haplotype having both PvuII and BstUI restriction sites on one chromosome was present only in the IVF group. Pregnancies from IVF were significantly rarer in patients who were homozygous for the PvuII polymorphism (P > 0.05). Our results suggest that genetic variability in the ESR has a role in the quality of the ovarian follicles as judged by the ovarian response to stimulation and may also affect implantation.