The treatment of metastatic castration-resistant prostate cancer has evolved rapidly over the past few years. First line treatment with one of the novel antihormonal drugs abiraterone or enzalutamide followed by chemotherapy with docetaxel is now standard of care. If a patient has disease stabilization on chemotherapy he undergoes a watchful waiting period and further treatment is only started at the time of disease progression. This trial tests the immediate use of the novel androgen receptor antagonist ODM-201 as maintenance treatment after chemotherapy aiming at prolonging radiographic progression free survival as compared to watchful waiting.