Introduction: With regard to transsexual developments, onset age (OA) appears to be the starting point of different psychosexual pathways.

Aim: To explore differences between transsexual adults with an early vs. late OA.

Methods: Data were collected within the European Network for the Investigation of Gender Incongruence using the Dutch Biographic Questionnaire on Transsexualism (Biografische Vragenlijst voor Transseksuelen) and a self-constructed score sheet according to the DSM-IV-TR (Diagnostic and Statistical Manual, Fourth Edition, Text Revision) criteria of Gender Identity Disorder (GID) and Gender Identity Disorder in Childhood (GIDC). One hundred seventy participants were included in the analyses.

Main outcome measures: Transsexual adults who, in addition to their GID diagnosis, also fulfilled criteria A and B of GIDC ("a strong cross-gender identification,""persistent discomfort about her or his assigned sex") retrospectively were considered as having an early onset (EO). Those who fulfilled neither criteria A nor B of GIDC were considered as having a late onset (LO). Participants who only fulfilled criterion A or B of GIDC were considered a residual (RES) group.

Results: The majority of female to males (FtMs) appeared to have an early OA (EO = 60 [77.9%] compared to LO = 10 [13%] and to RES = 7 [9.1%]). Within male to females (MtFs), percentages of EO and LO developments were more similar (EO = 36 [38.7%], LO = 45 [48.4%], RES = 12 [12.9%]). FtMs presented to gender clinics at an earlier age than MtFs (28.04 to 36.75). The number of EO vs. LO transsexual adults differed from country to country (Belgium, Germany, the Netherlands, Norway).

Conclusion: OA has a discriminative value for transsexual developments and it would appear that retrospective diagnosis of GIDC criteria is a valid method of assessment. Differences in OA and sex ratio exist between European countries.