Sex differentiation in humans depends on the presence of the Y-linked gene SRY, which is activated in the pre-Sertoli cells of the developing gonadal primordium to trigger testicular differentiation. Occasionally testicular formation can take place in subjects lacking a Y chromosome resulting in a 46,XX sex reversal condition. True hermaphroditism (TH) is a rare form of intersexuality characterized by the presence of testicular and ovarian tissue in the same individual. Genetic heterogeneity has been proposed as a cause of dual gonadal development in some cases and recently, hidden mosaicism was reported to cause TH in some 46,XX SRY negative patients. Here we report a TH case in which hidden mosaicism for the Y and X chromosome was detected by PCR and FISH in peripheral blood and gonadal tissue, supporting the fact that mosaicism may cause TH and that molecular analysis of gonadal tissue should be performed in all 46,XX cases.