Introduction: To our knowledge, there is only one report of lactic acidosis in a patient with prostate cancer and no such report in Japan. We describe a patient with terminal stage prostate cancer who showed marked elevation in lactate dehydrogenase (LDH) levels and lactic acidosis. Case Report: The patient was a 66-year-old man diagnosed with prostate cancer and bone and liver metastases. At the time of diagnosis, he was being treated with the best supportive care after chemotherapy and hormone therapy for prostate cancer. He was admitted to the hospital for nausea and loss of appetite. Laboratory tests showed elevated LDH levels (11,894 IU/L; LDH4, 23%; LDH5, 32%); pH, 7.402; pCO2, 13.2 mmHg; HCO3−, 8.0 mmol/L; and lactate level, 10.0 mmol/L. On the basis of the results, the patient was diagnosed with lactic acidosis. He showed tachypnea the day before his death. Conclusion: Elevation in LDH levels, and especially of LDH5, indicates acceleration of anaerobic glycolysis from tumor cells, leading to the accumulation of lactate. The patient’s tachypnea was possibly caused by lactic acidosis.