We report here the case of a patient with primary male ipogonadism, with small testes and deficient virilization of the external genitalia, but with 46XX, 45X0 karyotype. Hormonal determinations showed high LH and FSH and low testosterone levels. Ultrasonography confirmed the presence of small testes within the scrotum. Cytogenetic analysis demonstrated a female karyotype, with 90% 46XX, 10% 45X0 mosaicism. Using DNA probes for genes located on the Y chromosome, the presence of the "Sex-Determining Region" of the Y chromosome (SRY) was evidenced in the genomic DNA of this patient. By Fluorescence in Situ Hybridization (FISH), SRY locus was localized in the p terminal region of an X chromosome. SRY is the primary inducer of testis development; it acts as a transcription factor leading to a sequence of gene activations critical in the process of testicular differentiation and morphogenesis. A condition characterized by testicular development in subjects who lack a normal Y chromosome has been described; most of these patients are carriers of the short arm of the Y chromosome transferred to one of the two X chromosome, suggesting a form of X-Y paternal interchange. In our patient, the development of male gonade in the absence of an Y chromosome was explained by the demonstration of the SRY gene in an X chromosome.