Kallmann's syndrome (KS) is characterized by the association of hypogonadotropic hypogonadism and anosmia or hyposmia. Genetic defects have been observed throughout the KAL1 gene, located on the Xp22.3 region, in less than 50% of the patients. We report the molecular study of the KAL1 gene in 12 males with KS. PCR of the 14 exons of the KAL1 gene was performed on genomic DNA. PCR products of all exons were purified and sequenced. Three novel genetic defects were found. One patient exhibited a complete deletion of exon 5. The second presented a duplication of nucleotides 158-168; this insertion causes a termination codon (TGA) within the same exon. The third presented a mutation in exon 6, in which codon 262 changes from arginine to a stop codon. In the remaining nine individuals, no mutations were found. Three previously reported polymorphic changes were also documented. The deletion of exon 5 occurs within the region encoding the first fibronectin type III-like repeat of the KAL1 protein, this being the first KS patient who exhibits a complete deletion of a single exon of the KAL1 gene. The duplication of nucleotides in exon 1 is located in the conserved cysteine-rich N-terminal region that corresponds to the whey acidic protein motif, affecting the KAL1 protein either by interrupting the normal transcription or stopping the translation at the stop codon. The last novel mutation, a stop codon in exon 6, is located within the region encoding the first fibronectin type III-like repeat of the KAL1 protein. The absence of mutations in the majority of patients suggests the possibility of the existence of other genes involved or that in certain individuals the presence of various polymorphisms within the KAL1 gene could predispose to disease, as has been demonstrated in other pathological entities.