Currently, serum inhibin B levels, indicating testicular function due directly to its testicular origin, has been used increasingly in assisted reproductive units. Our aim in this prospective study was to evaluate the biological significance of inhibin B in gonadal dysfunctional males and the usefulness of inhibin B for the detection of male reproductive dysfunction. We included 52 oligoazoospermic and 20 normospermic men in this study. In our study serum inhibin B levels had statistically significant negative correlation with serum FSH and LH levels (P < 0.001, r: 0.781) and statistically moderate positive correlation with oestradiol levels (P < 0.005, r: 0.292). Inhibin B levels had significantly positive correlation with sperm count (P < 1.005, r: 0.851) and with testicular volume (P < 0.001, r: 0.466). Consequently, serum inhibin B level determination is a useful and non-invasive method for the evaluation of male gonadal dysfunction, taking into account its correlation with history, clinical examination, hormonal parameters, testicular volume, spermiogram and testicular biopsy.