This review summarizes observations on the background and potential clinical significance of interference with follicle stimulating hormone (FSH) regulation of human ovarian function. This interference may occur at the level of the pituitary by the secretion of FSH isoforms with reduced or absent bioactivity. In addition, interference with FSH may occur in the circulation, or within the ovarian follicular compartment. Although the full range of its significance remains to be elucidated, there are distinct indications that these mechanisms may be involved in normal ovarian physiology, as well as in abnormal response of the ovary to stimulation by endogenous FSH or by exogenously-administered gonadotrophin preparations. Moreover, recent advances in the determination of the structure-function relationship of FSH and FSH-receptor interaction, in combination with new developments in recombinant DNA technology, will allow the production of modified FSH- or FSH receptor-like molecules with altered bioactivity. The availability of FSH agonists and antagonists in the near future should provide a challenge for clinicians to improve treatment outcome and to find new indications for the use of these compounds.