The natural and treated course of Mycoplasma pneumoniae is generally satisfactory. Only a few cases with fatal outcome have been reported, mainly in immunosuppressed and debilitated patients. The case is reported of a healthy young woman admitted with acute respiratory failure, who developed diffuse interstitial fibrosing pneumonia followed by refractory hypoxemia. The diagnosis of Mycoplasma pneumoniae pneumonia was at first suspected from an elevated specific titer in the standard complement fixation test, and then confirmed in the immunofluorescence study by the presence of granular deposits (mycoplasma) in the macrophages of lung tissue. The functional impairments associated with this unusually malignant evolution are discussed.