The aetiology of cryptorchidism is for the most part unknown and appears to be multifactorial. Recently, a product of Leydig cells termed Leydig insulin-like hormone (INSL3) has been proposed as a putative trophic hormone of the first part of descent. Absence of Insl3 in male mice results in bilateral cryptorchidism and mutations involving this gene may be a cause of cryptorchidism in man. We sequenced both exons of the human INSL3 gene in 31 men who presented with idiopathic unilateral or bilateral cryptorchidism. The only sequence variant was an amino acid substitution in the C-peptide of the molecule. This change was also found in a control group of normal fertile men indicating that it is a polymorphism unrelated to the phenotype. These results suggest that mutations involving the human INSL3 gene are not a common cause of cryptorchidism in man.