Several recent papers indicate that the blood concentration of inhibin B measured by the dimeric assay is a marker of human spermatogenesis. The aim of the present study was to validate the dimeric assay in our hospital. For this purpose, we studied a population of 106 patients attending our infertility clinic. We found that serum inhibin B levels were positively correlated with the level of sperm production as reflected by the sperm concentration and negatively correlated with serum FSH levels. Serum inhibin B concentrations were found to be superior to FSH levels for discriminating between subgroups of patients with different levels of sperm production. For example, the differences in the inhibin B levels in severe oligozoospermia (< 5×106 sperm/ml) compared to non-obstructive azoospermia were more pronounced than the differences in FSH (p=0.0002 vs p=0.007, respectively). Overall, the results obtained in the present study were similar to those reported in other studies based on the same assay. Serum inhibin B levels in patients with non obstructive azoospermia were ten times lower than in patients with normal sperm concentrations. From a practical point of view, the measurement of serum inhibin B opens up new possibilities for the diagnosis and prognosis of many testicular disorders. Firstly, serum inhibin B determination should be performed whenever reproductive hormones are prescribed. In the particular case of azoospermia, serum inhibin B concentrations appear to be particularly useful to distinguish between obstructive and non-obstructive situations. Finally, according to a recent publication, a serum inhibin B cut-off value (> 40 pg/mL) could predict the success of testicular sperm extraction in patients with non-obstructive azoospermia. Serum inhibin B could therefore represent a unique non-invasive marker of focal hypospermatogenesis in men with non-obstructive azoospermia, who are candidates for intracytoplasmic sperm injection (ICSI) and a useful early marker to monitor possible recovery of spermatogenesis after chemotherapy or radiotherapy.