A 62-year-old male was admitted to our hospital after an abnormal shadow was pointed out by Chest X-ray in June, 1994, even though he was asymptomatic otherwise (first episode). The serum IgG antibody against Chlamydia pneumoniae was elevated up to 512-fold in the acute phase and decreased to 32-fold in the convalescent phase using the micro-immunofluorescence (MIF) test, indicating a C. pneumoniae acute infection. In fact, no other micro-organisms were detected. The patient recovered from the pneumonia without any treatment. He was admitted to our hospital again after having right chest pain in December, 1994 (second episode). An X-ray examination revealed a slight infiltration of the right lower lung field and pleural effusion. In this episode, he received therapy with carbapenem anti-bacterial agent and he recovered from the pneumonia 9 days after the administration of the antibiotic. The serum IgG titer against C. pneumoniae was elevated up to 1024-fold on admission, indicating mixed infection with bacteria and C. pneumoniae. It was concluded that both of these episodes indicated a spontaneous cure of the pneumonia which had developed from C. pneumoniae.