We report a rare case of giant prostatic hyperplasia treated by open surgery. A 70-year-old man was suffering from macrohematuria. Computed tomography revealed a markedly enlarged prostate measuring 580 mL. The serum prostate-specific antigen level was 9.430 ng/mL. Prostatic biopsy showed benign prostatic hyperplasia. We perfomed retropubic open prostatectomy, since macrohematuria continued and he was also suffering from lower urinary tract symptoms. The adenoma was completely enucleated in one piece. The removed specimen was 13 × 11 × 6 cm in size and weighed 475 g. Histological examination also demonstrated prostatic fibromuscular hyperplasia. This is the 15th-heaviest adenoma ever reported in English-language journals. Transurethral surgical techniques or other minimally invasive approaches are performed for patients with small to medium-sized prostates. However, open surgery is recommended for markedly enlarged prostatic hyperplasia.