Community-acquired pneumonia (CAP) due to Mycoplasma pneumoniae is usually mild, but some cases develop a severe life-threatening pneumonia. To investigate the clinical features of severe M. pneumoniae pneumonia in adults admitted to an intensive care unit, a multi-centre CAP surveillance study was performed. Among all hospitalized CAP cases between January 2000 and December 2004, there were 227 cases with M. pneumoniae pneumonia without the complication of other pathogens. A total of 13 of the cases required admission to an intensive care unit because of acute respiratory failure (ARF), and the remaining 214 cases (non-ARF) were low to moderately severe. The clinical features of ARF cases were compared with those of non-ARF cases. The underlying conditions in both types of case were identical, whereas clinical findings on admission clearly differed between the two groups. A regimen of an antibiotic effective against M. pneumoniae was begun on average at 9.3 days after the onset of symptoms in ARF cases, which was significantly later than for non-ARF cases (P<0.0001). However, two of the ARF cases progressed to respiratory failure despite the fact that adequate antibiotics were initially administered within 3 days after the onset of symptoms. All ARF cases received corticosteroids with adequate antibiotics, and their condition improved promptly. These results indicate that the clinical features, excluding underlying conditions, clearly differed between severe M. pneumoniae pneumonia and low to moderately severe pneumonia. The delayed administration of adequate antibiotics may contribute to the severity of M. pneumoniae pneumonia. Early corticosteroid therapy with adequate antibiotics should be considered.