STUDY QUESTION Does long-term exogenous testosterone administration result in polycystic ovarian morphology (PCOM), determined by (3D) transvaginal ultrasound (TVU) in female-to-male transsexuals (FtMs). SUMMARY ANSWER Long-term exogenous testosterone administration in FtMs does not result in PCOM determined by (3D) TVU. WHAT IS KNOWN ALREADY The role of androgens in the pathophysiology of polycystic ovary syndrome (PCOS) is still unclear. From animal studies, intra-ovarian androgens have been suggested to disturb folliculogenesis, through a pro-atretic effect on growing follicles. It remains debatable whether exogenous androgens induce PCOM in humans. In the past histomorphologic studies indicated that androgen administration in FtMs could cause PCO-like changes. However, ultrasound morphology is an established criterion for PCOS, TVU data of ovaries after prolonged androgen exposure are lacking. STUDY DESIGN, SIZE, DURATION Prospective, observational, case-control study, in an academic setting, performed in 2014-2015, including 56 FtMs and 80 controls. PARTICIPANTS/MATERIALS, SETTING, METHODS The study population consisted of adult FtMs treated with long-term testosterone, as part of their cross-sex hormone treatment, and scheduled for sex-reassignment surgery (bilateral salpingo-oophorectomy). Prior to the operation, under anaesthetics TVU measurements (3D transvaginal probe 3-9 MHz; HD11, Philips Ultrasound, Inc.) of the ovaries were performed. The control group consisted of females from a general population who underwent the same TVU and analysis. Antral follicle count (AFC) (3D) and ovarian volume (3D) were calculated using specialized software. PCOM was defined as AFC of 12 or more follicles (2-10 mm) in at least one ovary. MAIN RESULTS AND THE ROLE OF CHANCE Prevalence rates of PCOM were not significantly different in the FtMs compared to controls, determined by (3D) TVU: 32.1% (17/53) versus 30.7% (23/75), P = 0.87. LIMITATIONS, REASONS FOR CAUTION Testosterone levels in FtMs are supraphysiological, and may not be comparable to the testosterone levels in women with PCOS. However, we applied a unique and ethically acceptable opportunity of exploring the effects of androgens on human ovaries. WIDER IMPLICATIONS OF THE FINDINGS This first explorative study shows that long-term exogenous testosterone administration in adult women does not seem to induce PCOM determined by TVU. STUDY FUNDING/COMPETING INTEREST(S) None. TRIAL REGISTRATION NUMBER The trial was registered at the Dutch Trial Register (www.trialregister.nl), registration number NTR4784.