Elevated plasma follicle stimulating hormone (FSH) during the reproductive life is an early manifestation of ovarian ageing. The presence of elevated basal FSH in young, regularly menstruating women may represent a stage of menopausal transition consequent on premature ovarian failure. A total of 48 regularly menstruating, infertile women aged < 40 years, with high FSH and aged-matched controls with normal FSH underwent detailed monitoring of endocrine and follicle growth during one complete menstrual cycle. During the same cycle, detailed immunological screening was performed and the epidemiological features of all subjects were also reviewed. Subjects in the high FSH group had significantly higher basal FSH, luteinizing hormone (LH) and follicular phase LH concentrations. Despite their normal preovulatory oestradiol production, the high FSH group showed significantly slower follicular growth, smaller follicle diameter and lower luteal phase salivary progesterone. All these features have been described in older women during their menopausal transition. In addition, the prevalence of autoimmune antibodies was significantly higher in the high FSH group. This study suggests that infertile women with elevated FSH are in their perimenopause despite having regular ovulatory and apparently normal cycles. An autoimmune basis is suggested as a factor underlying their premature ovarian failure. Further endocrinological and autoimmunological follow-up is recommended.