A 29-year-old woman developed fever and sore throat 14 days before admission. Six days later she noted the onset of a rash and ten days later she noted a dry cough. The chest X-ray film showed no abnormal shadow, but a chest CT scan showed a diffuse ground-glass shadow and a 67Ga-scintigram showed abnormal lung uptake. The level of anti-cytomegalovirus antibody of the IgM class was high; cytomegalovirus mononucleosis was diagnosed and cytomegalovirus pneumonia was suspected. Cytomegalovirus DNA was detected in bronchoalveolar lavage fluid by the polymerase chain reaction method. We known of no previously reported case of cytomegalovirus mononucleosis in which cytomegalovirus DNA was found in bronchoalveolar lavage fluid by the polymerase chain reaction method.