Studies on diagnostic subtypes of gender identity disorder (GID) or gender incongruence (GI), comorbidity and treatment outcome show considerable variability in results. Clinic/country specific factors may account for the contradictory results, but these factors have never been studied. This article is the first of a series reporting on a unique collaborative study of four European gender identity clinics (the European network for the investigation of gender incongruence [ENIGI]). Here, we present the diagnostic procedures of the four clinics (Amsterdam, Ghent, Hamburg, and Oslo), the standard battery of instruments, and the first results regarding applicants with GI who seek treatment. Applicants in the four clinics did not differ in living situation, employment status, sexual orientation, and age of onset of GI feelings. However, the Amsterdam and Ghent clinic were visited by a majority of natal males, whereas Hamburg and Oslo see more natal females. Male applicants were older than female applicants within each country, but female applicants in one country were sometimes older than male applicants in another country. Also, educational level differed between applicants of the four clinics. These data indicate that certain sociodemographic and/or cultural characteristics of applicants have to be taken into account in future studies.