This ultrahigh-field MRI study gives us the unique opportunity to measure changes in brain function and structure induced by long-term opposite-sex steroid hormone administration in transsexual subjects. Our aim is 1. to prove the influence of high-dose, long-term opposite-sex steroid hormone treatment on functional brain response in transsexuals. 2. to investigate the influence of hormone treatment on resting state functional connectivity and brain morphology. 3. to investigate differences between transsexuals and healthy control subjects in brain function and functional connectivity, brain morphology and structural connectivity. In this longitudinal study, transsexuals and healthy control subjects will undergo three 7 Tesla ultrahigh-field MRI scan sessions: 1. baseline (before hormone treatment), 2. after 4 weeks of treatment and 3. after 4 months of treatment.

Inclusion criteria for transsexuals are:

• DSM-IV diagnosis of Gender Identity Disorder (DSM-IV: 302.85, 302.6; ICD-10: F64.9, F64.8) by a structured clinical interview (SCID)
• general health based on history, physical examination, ECG, laboratory screening, SCID
• willingness and competence to sign the informed consent form

Exclusion criteria for transsexuals are:

• severe neurological or internal diseases
• steroid hormone treatment within 2 months prior to inclusion (including birth control pill, phytohormones)
• treatment with psychotropic agents such as SSRIs
• any implant or stainless steel graft
• abnormal values in routine laboratory screening or general physical examination
• current substance abuse (determined using drug screening at the screening visit)
• pregnancy (determined at screening visit and first MRI scan)
• failure to comply with the study protocol or to follow the instructions of the investigating team.

Inclusion criteria for healthy controls are:

• general health based on history, physical examination, ECG, laboratory screening, SCID
• willingness and competence to sign the informed consent form

Exclusion criteria for healthy controls are:

• severe diseases
• any implant or stainless steel graft
• steroid hormone treatment within 2 months prior to inclusion (including birth control pill, phytohormones)
• abnormal values in routine laboratory screening or general physical examination
• current substance abuse (determined using drug screening at the screening visit)
• pregnancy (determined at screening visit and first MRI scan)
• failure to comply with the study protocol or to follow the instructions of the investigating team

Transsexuals urging sex reassignment in a clinical setting will be enrolled at the Unit for Gender Identity Disorder at the General Hospital in Vienna. 20 female-to-male (FtM) and 20 male-to-female (MtF) transsexuals will be asked to undergo this fMRI, MRI, DTI and resting-state functional connectivity study. Transsexual subjects are aged between 18 and 50 years old and free from hormone-treatment at baseline.

40 healthy control subjects, matched for age, sex and sexual orientation to transsexual subjects will be recruited.