Background <p>Hereditary Hemorrhagic Telangiectasia (HHT) is a rare vascular disorder with mucocutaneous telangiectasia. Hormones are hypothesized to play a role in reducing bleeding in these patients; however, clear evidence is currently lacking. The aim of this study was to further investigate this hypothesis by assessing the impact of altered hormonal status on HHT severity.</p> Methods <p>This cross-sectional electronic questionnaire, available in English and German, was distributed with the support of various self-help groups. It included sections on the general medical history of HHT and the influence of hormones on disease symptoms. Data were collected between March 2016 and August 2017.</p> Results <p>Of the 477 respondents with HHT, 326 were female (68%). With increasing age, female patients experienced more severe epistaxis compared to male patients, with a mean age of 54 years (standard deviation ± 11.5 years, range [28,83]). Of the 228 female patients who reported having taken hormones, 73% used them for contraception and 57% for hyper- or dysmenorrhea. Regarding hormone intake, 52% of the patients taking combined estrogen and progestin therapy experienced less epistaxis. Most female patients stated an increase in epistaxis during puberty (41%) and pregnancy (39%), predominantly in the second trimester.</p> Conclusions <p>In HHT, variation in levels of hormone concentrations seem to contribute to symptom severity. Off-label use of hormonal products in women with HHT may be considered.</p> Trial registration <p>Clinicaltrials.gov: NCT02690246, Registration Date: 2016–02-09.</p>