We report an unusual case of bilateral, synchronous breast cancer in a male patient who had a history of estrogen therapy for prostate cancer. A 64-year-old Japanese man was diagnosed with T1N0M0 prostate cancer and received a total prostatectomy. Twenty months after the resection, the patient developed multiple bone metastases, and received radiation therapy, systemic chemotherapy, and hormone therapy for 15 months. After completing this treatment, he was diagnosed with T1N0M0 primary breast cancer in his left breast and underwent a modified mastectomy. Five months after the mastectomy he received systemic chemotherapy followed by estrogen therapy because of the progression of prostate cancer. Three months after this treatment, he was diagnosed with T1N0M0 primary breast cancer in his right breast. To the best of our knowledge, this is a rare case of synchronous bilateral male breast cancer following hormone therapy for prostate cancer.