Androgen insensitivity syndrome (AIS) is a rare X-linked disorder, caused by mutations in the androgen receptor gene (AR), associated with a variety of phenotypes in 46,XY individuals. We studied two 23 year-old twin-sisters with female social sex referred due to primary amenorrhea, who exhibited bilateral palpable gonads in the inguinal region and a 46,XY karyotype. The uterus was absent in pelvic sonograms. Basal LH levels were elevated (35 and 42 U/L), with normal FSH (7.9 and 7.8 U/L) and high testosterone levels (1330 and 1660 ng/dl). The molecular analysis identified a missense mutation in exon 5 of AR gene that changed a proline to an alanine at position 766 of the protein. Proline 766 is a highly conserved amino acid in the AR of several species and is located in the androgen binding domain.