Randomized phase II study to investigate the efficacy, safety and tolerability of ZK 230211 (100 mg vs. 25 mg) as second-line endocrine therapy for postmenopausal women with hormone receptor-positive metastatic breast cancer.Once the cancer has spread beyond the lymph nodes to areas such as e.g. the skin, soft tissues, lung, and liver it is called metastatic breast cancer. Patients who have been diagnosed with metastatic breast cancer that has progressed since their previous cancer treatment and that cannot be removed completely by surgery are eligible to be treated within this trial.Treatment with a new drug called Progesterone Receptor Antagonist ZK 230211 (ZK PRA) targets the progesterone receptor which may be expressed on breast cancer tumour cells. Therefore only patients with this progesterone receptor on their tumour cells can be included in this study.Progesterone receptor antagonists (including onapristone) have already shown efficacy in postmenopausal women with advanced breast cancer (Klijn et al. 2000). This phase II study investigates the efficacy (proof of concept), safety and tolerability of ZK PRA at two dose levels (25 mg and 100 mg) before initiating pivotal phase III trials.