This is a multi-center, open-label, phase I/IIa trial, dose-escalation study with a 3 + 3 cohort design to determine the recommended dose of estetrol for the treatment of patients with advanced breast cancer. After completing phase I part of the study (i.e. 4 weeks of treatment), patients will receive further treatment for 8 weeks at their individual phase I dose level (phase IIa part of the study).