Steroids, including estrogens, participate in sleep regulation. For example estrogen replacement therapy improved sleep quality in postmenopausal women. Patients, who undergo a cross-gender hormone therapy, receive high doses of estrogens. The effects of this treatment on sleep are unknown. To clarify this issue, we examined seven male to female transsexual patients (age range 31-44 years, mean±SD 35.9±4.2 years). The patients spent two nights on 2 separate occasions in our sleep laboratory. The first night of each session served for adaptation to laboratory conditions. In the second night sleep electroencephalogram [EEG] was recorded from 2300h to 0700h. The first examination was performed before and the second about 3 months after initiation of cross-gender hormone therapy with a dose of 80-100mg estrogen applied every 2 weeks. Additionally patients were treated with a starting dose of the anti-androgen cyproteronacetate of 100mg/day and after about 6 weeks with a maintenance therapy of 50mg/d in order to suppress androgenic effects. Statistical analysis was performed with the Wilcoxon rank test. Under this estrogen therapy we found a significant increase in stage 1 sleep during the whole night (at baseline [b]: 33.29±9.94min; treatment [t]: 51.57±24.26min; p<0.05) Beta activity in nonREM sleep was significantly increased (p=0.02) during hormone therapy compared to before treatment. Other sleep EEG parameters showed no significant changes. Administration of estrogen and anti-androgens in male to female transsexual patients had only a small influence on sleep EEG, with an increase in the duration of shallow sleep.