A 33-year-old homosexual Japanese man who admitted to having sex with men presented with a two-week history of dyspnea and fever. Chest imaging showed diffuse pulmonary frosted-glass-like shadows. A blood test revealed positive HIV antibodies with a CD4 cell count of 66/μL. Bronchoalveolar lavage identified pneumocystis. Although the patient exhibited a transient response to anti-pneumocystis treatment and mega-dose steroid pulse therapy, he eventually died from respiratory failure. An autopsy suggested massive cytomegalovirus and pneumocystis pneumonitis. The pulmonary co-infection with cytomegalovirus may have been worsened by the use of mega-dose steroids, and such therapy should be avoided in patients with a high HIV viral load and low CD4 count.