Objective To summarize the characteristics of clinical manifestation of bone flare after the treatment with new endocrine therapy in patients with metastatic castration-resistant prostate cancer (mCRPC) in order to evaluate the curative effect of patients properly and determine the reasonable treatment strategy.Methods We retrospectively analyzed the clinical data of two patients with mCRPC performed "bone flare" defined as PSA decline and bone metastases progression in the initial treatment with new endocrine therapy in Urology Department of Peking University First Hospital,and analyzed the clinical characteristics and treatment methods with the relative literature.Case 1,a 79-year-old man,presented with frequent urination and prostate-specific antigen (PSA) was 115.900 ng/ml,was diagnosed as prostate cancer (cT3N0M1) with bone metastasis.After androgen deprivation therapy of 24 months,PSA elevated and multiple bone metastases progressed.The patient was diagnosed with mCRPC and then began the treatment of enzalutamide.Case 2,a 62-year-old man,complained about emaciation and frequent urination,was diagnosed with prostate cancer(cT4N1M1)with bone and lymph metastases.After androgen deprivation therapy of 22 months,PSA elevated and multiple bone metastases progressed.The patient was diagnosed with mCRPC and then began the treatment of abiraterone.Results Case 1 was treated with enzalutamide and 2 months later PSA decreased from 133.400 ng/ml to 5.530 ng/ml,while bone scan showed multiple bone metastases,part of which was newly metastatic lesions.6 months later,the number of metastatic lesions kept stable,and part of lesions presented metabolism decrease.8 months later,the number of metastatic lesions began to decrease.1 year later,the patient started to receive chemical therapy because of the progression of the disease.After 5 cycles of chemotherapy,PSA progression occurred and chemotherapy was stopped.Liver failure and disseminated intravascular coagulation caused death in June 2016.Case 2 was treated with abiraterone and 2 months later PSA decreased from 54.820 ng/ml to 3.580 ng/ml,while bone scan showed multiple bone metastases,part of which was newly metastatic lesions.6 months later,the number of metastatic lesions began to decline.10 months later,the number of metastatic lesions kept stable.The treatment of abiraterone was continued so far and the patient was in a stable condition.Conclusions Enzalutamide and abiraterone,two new endocrine therapy,are determined as preferred methods for the treatment of mCRPC.The bone scanning is required to evaluate the possibility of "bone flare" which is defined as PSA decline and bone metastases progression in the initial treatment.These patients should be evaluated to make appropriate clinical decision.