Gender transformations are normatively understood as somatic, based on surgical reassignment, where the sexed body is aligned with the gender identity of the individual through genital surgery - hence the common lexicon 'sex change surgery'. We suggest that the UK Gender Recognition Act 2004 challenges what constitutes a 'sex change' through the Act's definitions and also the conditions within which legal 'recognition' is permitted. The sex/gender distinction, (where sex normatively refers to the sexed body, and gender, to social identity) is demobilised both literally and legally. This paper discusses the history of medico-socio-legal definitions of sex have been developed through decision making processes when courts have been faced with people with gender variance and , in particular, the implications of the Gender Recognition Act for our contemporary legal understanding of sex. We ask, and attempt to answer, has 'sex' changed