STUDY QUESTION: What is the semen quality in trans women at time of fertility preservation, prior to the start of gender-affirming hormone treatment? SUMMARY ANSWER: Before the start of gender-affirming hormone treatment, semen quality in trans women was already strongly decreased compared to the general population. WHAT IS KNOWN ALREADY: Hormone treatment for -trans women (birth-assigned males, female gender identity) consists of anti-androgens combined with estrogens in order to achieve feminization and it is accompanied by a loss of reproductive capability. Trans women can opt for semen cryopreservation prior to their medical transition to retain the possibility to parent genetically related offspring. Post-thaw semen parameters determine which ART can be used. Knowledge of semen quality and the factors negatively influencing semen parameters in trans women are important to improve semen quality before fertility preservation. STUDY DESIGN, SIZE, DURATION: A retrospective cohort study was performed between 1972 and 2017. In total, 260 trans women were included for this study. Due to the study design, there was no loss to follow-up or attrition. PARTICIPANTS/MATERIALS, SETTING, METHODS: We studied the quality of the preserved semen in trans women, prior to their medical transition, who visited our gender clinic. Semen parameters were collected, as well as data on age, alcohol consumption, smoking, cannabis use, BMI, previous use of estrogens or anti-androgens and endocrine laboratory results. Semen parameters were categorized using reference values for human semen of the World Health Organization (WHO) and compared with data from the general population. Logistic regression analyses were performed to analyze the extent to which factors known to have a negative impact on semen quality in the general population explained the impaired semen quality in the cohort. MAIN RESULTS AND THE ROLE OF CHANCE: The cohort consisted of 260 trans women between the age of 16 and 52 years. Semen quality in trans women was significantly decreased compared to WHO data from the general population. In total, 21 trans women had an azoospermia and median semen parameters for the remaining trans women and the general population, respectively, were as follows: volume 2.7 and 3.2 ml (P < 0.05), sperm concentration 40 and 64 million/ml (P < 0.05), total sperm number 103 and 196 million (P < 0.05) and progressive motility 41% and 57% (P < 0.05). Smoking (odds ratio (OR) 2.35 (95% CI 1.06-5.21)) and a higher age at time of fertility preservation (OR 1.04 (95% CI 1.00-1.08)) were found to correlate with an impaired progressive motility. Twelve trans women reported to have used anti-androgens and estrogens, and all had discontinued for at least 3 months prior to the first attempt for semen cryopreservation. No correlation was found between previous gender-affirming hormone use and decreased semen parameters. The median post-thaw total motile sperm count was 1.0 million per vial (interquartile range 0.1-3.1) and in only 26.4% of thawed semen samples was the quality adequate for a minimally invasive IUI. LIMITATIONS, REASONS FOR CAUTION: Limitations include the retrospective design and insufficient data on transgender-specific factors, such as bringing the testes into the inguinal position (tucking), wearing tight underwear and low masturbation frequency. WIDER IMPLICATIONS OF THE FINDINGS: Semen quality in trans women was decreased compared to the general population, which could not be explained by known risk factors, such as BMI, alcohol consumption, cannabis use, gender-affirming hormone use or abnormal endocrine laboratory results. Although a negative impact of smoking was observed, it was insufficient to explain the overall decreased semen quality in this cohort. Since low pre-freeze semen quality results in an even lower post-thaw semen quality, the majority of trans women and their female partner or surrogate may need an invasive and burdensome treatment to establish a pregnancy. STUDY FUNDING/COMPETING INTEREST(S): For this study, no external funding was obtained and there were no competing interests.NA.