Kallmann syndrome is defined by the association of hypogonadotropic hypogonadism and anosmia, for which three modes of transmission have been described: X-linked, autosomal recessive and autosomal dominant. The KAL1 gene, responsible for the X-linked form of the disease, has been isolated and its intron-exon organization determined. We report sequence analysis using PCR-direct sequencing method of the entire coding region and splice site junctions of the KAL1 gene in three males with Kallmann syndrome. We found a novel mutation in one case and no mutation in the other two cases. The mutation consisted of a C to T substitution in exon 1 converting codon 66 (CAG) encoding glutamine into a termination codon (TAG)/(Q66X). As a consequence of this mutation, the function of the KAL1 protein consisting of 680 amino acids was severely truncated so as to be consistent with Kallmann syndrome. As only this patient had unilateral renal hypoplasia among the three cases, this would suggest the existence of KAL1 gene mutation in this abnormality.