Authors report on a case of prenatal diagnosis of hydrocolpos in a genetically male subject. Postnatal sonography and endoscopy confirmed the presence of a urogenital sinus and a vaginal cavity. Despite these findings, karyotype unexpectedly revealed a 46 XY, SRY positive, male subject. Even in the presence of an established diagnosis of fetal hydrocolpos, caution should be taken before conveying information to the parents about the fetal sex. Fetal karyotyping needs to be obtained whenever possible.