Men with idiopathic infertility and normal serum FSH levels, candidate for treatment with FSH, will be recruited. Men with a sperm DFI > 15% will be included in the trial if carriers of the homozygous Asn/Asn or Ser/Ser at aminoacid position 680. The FSHR genotype will be assessed centrally and the physician will only receive the information whether the patient is eligible for entering the trial (i.e. homozygous) but both the physician and the patient will remain blind to the genotype. Human recombinant FSH (Gonal-f, Merck Serono) will be self-administered sc at the dose of 150 IU every other day for 12 weeks, followed by 12 weeks of observation (follow up). Changes in sperm DFI will be the primary end point and compared between the two arms. In addition, the effects on pregnancy rate and other clinical and hormonal parameters will be evaluated.