Male factor infertility is responsible for about 50% of cases of involuntary couple infertility and remains idiopathic in about half of the cases. At present, there are no consistently effective treatments for male idiopathic infertility. Since follicle-stimulating hormone (FSH) is fundamental for spermatogenesis, recombinant hFSH is empirically used for male infertility treatment. The response to FSH, however, is highly variable and while sperm parameters improve in some patients, about 50% of the subjects do not clearly respond to FSH. Several studies were performed in the past and a recent Cochrane meta-analysis showed that FSH treatment of male idiopathic infertility overall significantly improves pregnancy rate. Nevertheless, no predictive marker of response to FSH, allowing a stratified therapeutic approach, was identified so far.