Isolated FSH deficiency due to a mutation in the FSHbeta subunit is characterized by an extremely low serum FSH concentration. We report a patient who presented with an FSH of 0.8 mIU/ml and infertility associated with anovulation. Endocrinological assessment and immunohistochemistry revealed that a granulosa cell tumour was secreting inhibin B and suppressing FSH; however, LH and estradiol were within their normal ranges. Upon removal of the tumour, inhibin B decreased and FSH levels rose to normal values. The patient subsequently conceived and delivered successfully. Based on this case and on those previously described in the literature, we suggest that inhibin B levels should be evaluated in anovulatory patients having a clinical presentation consistent with functional hypothalamic amenorrhoea and very low to normal values of FSH.