A 73-year-old man underwent 80 mg BCG intravesical instillation therapy for high-grade pT1 bladder cancer. In addition, after receiving six infusions, he underwent BCG maintenance therapy every three months for three weeks, which was administered after using a urethral bougie to dilate his urethral stricture. On the same day, he was referred to our hospital because of fever and impaired consciousness. Upon examination, fever and decreased blood pressure were noted and the infected area was unclear; however, computed tomography (CT) revealed an increase in the perirenal fat concentration. BCG sepsis was diagnosed based on these findings and the patient's medical history of BCG administration after receiving the urethral bougie. The progress of treatment after hospitalization is shown in Figure 1. He was administered antituberculosis drugs (INH, 300 mg/day; RFP, 450 mg/day; and EB, 750 mg/day), an empirical nonspecific antibiotic, and an antidisseminated intravascular coagulation drug (thrombomodulin alfa) for BCG sepsis and disseminated intravascular coagulation after various tuberculosis tests were performed. Steroids were not administered because of the possibility of M. bovis infection. Ventilator management was performed for airway obstruction caused by oral bleeding, and dialysis management was required because of acute renal failure. Steroid pulse therapy was started because the fever persisted. CT on day 12 revealed findings suggestive of interstitial pneumonia. A diagnosis of interstitial pneumonia due to BCG was established based on the patient's presentations as follows: fever, decreased oxygenation, and no history of imaging findings of interstitial pneumonia. Steroid pulse therapy was administered (methylprednisolone, 1000 mg/day) for three days, and prednisolone was administered at a maintenance dose of 80 mg/day (as per 1 mg/kg body weight). The fever and inflammatory response reduced, and dialysis was no longer required. Additional steroid pulse therapy (methylprednisolone, 1000 mg/day for three days) was administered on day 19, and mechanical ventilation was discontinued on day 20. The dose of prednisolone was gradually reduced from 80 mg, and the symptoms did not recur. He was discharged on day 51. EB was stopped on day 64, and INH and REP were stopped on day 322; the symptoms have not recurred since then.