The full etiology of transsexualism is still unknown. However many factors, like biological and environmental, are being suggested as possible explanations to the cause of this entity.

We intend to revisit the major etiological theories of transsexuality, based on the description of two cases occurring in first-degree relatives.

A mother and her daughter both diagnosed with Gender Identity Disorder are followed at Multidisciplinary Clinical Sexology Group in our facilities since 2011.

This case is related to a young girl who in her childhood felt and behaved himself as a boy by adopting male gender stereotypes. She has lived with distress during the development of female secondary sexual characteristics, and the age of 17, after identifying his problem decided to tell his mother. However our patient founded that her mother, with 38 years old, was already resigned by living with a sex gender that didn't recognize as her own. In the end they decided to confront the social and family environment, joining both in the process of sexual reassignment.

The transsexualism is a complex phenomenon. The occurrence of two cases of transsexualism in a mother and daughter highlights the importance of consider and extend the knowledge about the biological aspects of gender identity disorder.

It is relevant to communicate the cases occurring within the same family to contribute to the etiology investigation in this area. In the literature the rare cases described of family transsexuals are among brothers, which makes this case between mother and daughter special.