Anti-Müllerian hormone (AMH) also known as Müllerian inhibiting substance or factor, is a Sertoli cell-secreted glycoprotein responsible in male embryos for Müllerian duct regression. However, its role in adults remains unknown. AMH seminal concentrations have been evaluated using an enzyme-linked immunoassay in three groups of young men: group 1, fertile donors (n = 18); group 2, obstructive azoospermia (n = 9) after vasectomy or associated with deferent duct agenesia; and group 3, non-obstructive azoospermia with spermatogenesis deficiency and normal karyotype (n = 23). AMH was present in seminal plasma of most fertile donors at concentrations ranging from undetectable (<3.5 pmol/l) up to 543 pmol/l (geometric mean: 153 pmol/l), higher than the serum level (range <3.5 up to 67 pmol/l, geometric mean: 10.7 pmol/l, n = 13). Seminal AMH concentrations were undetectable in all obstructive azoospermic patients, confirming its testicular origin. In non-obstructive azoospermia (group 3), seminal AMH concentration was lower (range <3. 5-68.5 pmol/l, geometric mean: 17 pmol/l) than in fertile donors (P < 0.003) without correlation with plasma follicle stimulating hormone values. In group 3, comparison of seminal AMH concentration and the results of histological analysis of testicular biopsies revealed that undetectable AMH found in 14 cases was associated in 11 of them with lack of spermatozoa, while detectable concentrations of AMH (10-68.5 pmol/l) found in nine cases were associated in seven of them with persistent spermatogenesis. In the adult, AMH is secreted preferentially towards the seminiferous lumen. Although its relationship with spermatogenesis requires further investigation, our results suggest that seminal AMH may represent a non-invasive marker of persistent hypospermatogenesis in cases of non-obstructive azoospermia which may indicate the likely success of testicular spermatozoa recovery before intracytoplasmic sperm injection.