We investigated 90 patients with Mycoplasma pneumoniae (M. pneumoniae) pneumonia. Forty-four patients were men, 46 were women and the mean age was 43.1 years old. Twenty-nine patients were smokers and 28 had underlying diseases. As for diagnostic method, 16 were culture positive, 71 had a fourfold increase in antibody titer to M. pneumoniae, and 3 were both culture positive and had a fourfold increase in antibody titer. Regarding the degree of severity, 21 patients were classified as severe according to Japanese Respiratory Society diagnostic criteria, 11 patients were diagnosed as severe according to American Thoracic Society diagnostic criteria. Partial pressure of arterial oxygen (PaO2) of 18 patients were <60mmHg, 5 patients were under mechanical ventilation, and 3 patients died. Three of 16 patients treated with only beta-lactum antibiotics recovered. The 3 patients who died were M. pneumoniae culture-positive and two patients had polymicrobial infections. Severe pneumonia associated with Mycoplasma pneumoniae infection is not unusual. If a rapid diagnosis kit or culture method of M. pneumoniae pneumonia is not introduced, the pathogen might be unknown in cases of rapid death due to M. pneumoniae pneumonia. These data suggest that the mortality rate of M. pneumoniae pneumonia might be underestimated without these detection tests.