The Transgenderism reference article from the English Wikipedia on 24-Jul-2004
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Transgenderism

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Transgenderism is often used as synonymous with transgender. However, transgenderist often (but not always) refers specifically to people who do life in another gender role than their birth sex, use Hormone replacement therapy but who do not wish to have sex reassigment surgery, and often even oppose other people having sex reassigment surgery or at least genital reassigment surgery.

Transgender people therefore often feel that being called "transgenderist" is wrong or even discriminating.